"You'd think, wouldn't you! ... I will of course not in any way comment on who or what might be teaching the children to be, haha, unprofessional. I will however comment that if any of you choose to make illegal decisions, whether in my general direction or otherwise, I will always be available to offer advice and I will not report you to the DMLE about it, because I am an adult and you are eleven."

(Some people might consider this statement to be unwise on grounds that it might incentivize murder attempts. Septimus Weasley does not consider the number of times a week people attempt to murder him to be a relevant input to his decision-making, on account of it never works, and instead makes decisions based on, e.g., what might give baby Slytherins a chance to grow up into better people.)

"You might not like my advice, of course, as I'm given to understand it is incorrigibly Gryffindor, but there you are. Any other questions before we begin with the legally mandated red sparks charm?"

"Were you going to introduce yourself?" wonders Dolohov, very dryly.

"Oh. Huh. Totally assumed all of you would obviously have heard of me. Septimus Weasley."

Wilkes gasps. "Oh my gosh you're the guy who won a duel with Arct -"

Mulciber shushes her.

" - erm. Who did not. Famously. Win an unwinnable duel. Because that uh. Is impossible?" 

"We've heard of you, yeah," mutters Avery.

She, meanwhile, lets her genuinely-not-recognizing-him face show.  ...Which is, really, not that different from her normal face, but the fact that it's busy turning its sharp gaze upon all the other children while they have their reactions to a famous duelist may be somewhat informative.

Karina Dolohov: Trying so hard not to have a reaction. Slightly intimidated and mad about it. 

Annette Wilkes: Trying somewhat less hard not to be starry-eyed. She knows this is uncool of her and is trying not to but she would like to extract all of Weasley's coolness and damn the consequences because if she could be that cool she would not need to care about consequences actually.

Tiberius Mulciber: So, so suspicious. Maximum suspicious. Would totally have been attempting an accidental murder if not for how now it would just be embarrassing. Is going to second- and third-guess one hundred and fifty percent of all things he is told in this class as potentially contaminated by the enemy.

Philip Avery: Like fifty percent that suspicious, which is honestly still a lot. Also, kind of scared that Mulciber is going to do some bullshit and then he'll get blamed somehow.

Evan Rosier: Mostly just terrified.

Well, that's good.

Mostly.

(Really, Tiberius?  Really?  You're that incapable of forethought and risk assessment?)

Rosier's mostly-sourceless terror...She would prefer Not That, it's loud and kind of annoying, and is rather inclined to try and notepass a "Relax, he's not going to kill you unless you start something first" if she can find the leeway to do so, though she doesn't expect that leeway to be readily available.

...She's very curious how Septimus is reacting to the class's reactions, though she's probably missed her window...

"I do have one question, actually; when and where will you be available for the consultations you mentioned, such that we can, as Slytherins of breeding and class, be sure to avoid mistakenly doing something like showing up?  As, surely, we all agree that any such rumors of us doing would be so radically implausible as to dismiss out of hand, no matter the worth of an opportunity to consult the expertise of a duellist so puissant his greatest victory is too impossible to be contained within speakable words."

His reaction, both to the class in general and to Ophelia's little speech, is mostly bemused blinking.

"My office hours are 1-2 Monday through Thursday."

"Thank you, Professor."

She makes a note.

"Certainly. I look forward to... definitely not ever seeing any of you in my office on purpose, I suppose. Now then! It is my solemn legal duty, apparently, to make sure that you all learn to cast vermillious, the red sparks charm. The function of this charm is to make a bright magical light, which will set off an alarm anywhere a suitable ward has been placed so that, if you need help, an adult wizard can be alerted to come rescue you. Such wards can be found everywhere on the grounds of Hogwarts Castle and within a few hundred yards of most wizarding homes. Does anyone know the other two places on the Isles where that's true?"

If she had to hazard a guess, the general environs of Diagon Alley - though perhaps that is 'most wizarding homes' - and, what, mundane London?  There's certainly enough wizarding infrastructure afoot...  But she's not going to offer this answer, being as she's guessing.

Slytherins are not known for their collective willingness to enthusiastically raise their hands and make uncertain guesses. No one else offers an answer either.

Professor Weasley waits a few beats, considering the silent pile of them, shrugs, and calls on Evan, who looks like he needs encouraging.

"Um! Um, platform nine and three-quarters? And, uh, the... the ministry of magic?" Evan hazards, with a tiny but detectable preemptive flinch.

He is definitely not going to spend his winter holidays doing murders about the traumatized baby Slytherins. He specifically promised Dumbledore he would not do that. He is a responsible adult who keeps his promises. For fuck's entire sake, though, why. 

Evan gets a warm, encouraging smile for his efforts, though not before a brief flash of quickly-buried concern. "Excellent guesses, Mr. Rosier, thank you! You are three-quarters correct. Platform 9 3/4 indeed has its own spark detection wards, and has since, I believe, nearly its creation, in the 1700s or thereabouts but don't quote me on that bit, I am not a historian. The Ministry of Magic does not, as it is a very boring place that children do not go to very often, but it's been regularly proposed and I suspect it'll get into MLE's budget any decade now. Any other ideas?"

Nobody volunteers, still.

"Right. We'll work on that. The other dedicated non-residential ward currently in existence is, for historical reasons, on the island of Azkaban. You should, regardless, never, ever go there if you can possibly avoid it, but in the incredibly unlikely event that this is ever relevant to your life, remember that the Aurors do monitor that ward and they will probably lecture you if you're there for a stupid reason but they will not leave you there." He waves his wand at the board, upon which is now written

WHERE RED SPARKS WORKS:

- HOGWARTS

- RESIDENTIAL AREAS (DIAGON ALLEY, HOGSMEADE, YOUR OWN HOUSE)

- PLATFORM 9 3/4

- AZKABAN

"Consider writing this down," he adds, in the general direction of folks who are not already taking notes. "I am going to ask you this one again next week. Any other questions before we start in with the wand motions?"

"Is there a way to determine if you are in a warded area without potentially spuriously triggering the ward?"

...She's going to have to read up on this 'Azkaban' place.

Huh. Usually people just tell their kids where the edges are. "Y..es but also for your purposes not really?"

This particular child is not going to find that answer at all satisfying, is she.

"... so, the thing is, it will show up on most analysis charms, yeah, but most people can't cast any of those until OWL year. It seems to me that there's no reason anyone would make spark detector wards intentionally hard to detect like they might, say, an intruder tripwire, though. If you're not just academically curious maybe ask Flitwick? He knows all manner of random obscure things that are completely useless except for the exactly one specific time they are incredibly useful."

(He says this last thing with great fondness.)

"I see."

She adds 'Flitwick: Vmls alarm wards?  Analysis charms.' to the small notebook she keeps in her robe's front pocket.  (Using a pencil.)

"I must admit that if the Ministry spends funds and hours enough for a ward the size of Britain - and Ireland? - entire, to track underage magic, but does not then include their designated emergency signal as a special case within the warding it would trigger, I am rather unimpressed with them."

"Valid complaint if that were true, which it is not. Regrettably I am legally not allowed to tell you how the Trace does work because you are underage."

And with that he'll start in on the explanation of how to actually cast vermillious, before he finds himself having to defend the Ministry's decisions in any detail, which does not sound like a fun activity.

Hmm.  Interesting.

Oh, the spell?  No, no.  The information that the Trace isn't location-based.

Apparently.

...She's rather quick to get Vermillious down, even though this is the one spell she definitely didn't dare practice in advance.  The actual process of learning it involves a surprising amount of diagramming, if you're paying attention to how she does it - well, if there's any particular wand motion involved.  She draws that out in isometric perspective, from a particular starting-point much akin to how one holds a conductor's baton, and marks the points of the incantation along the points of the wand motion.

Only once she is sure of the structure of the spell does she begin to practice it, going through the motions at half-speed, then reciting the syllables, then, carefully, with a dummy wand (read: an unsharpened #2 pencil) combining the two until she's at 'full speed' - she's going to be working on that, incidentally; pronouncing faster and moving quicker is less useful that not needing to pronounce or twiddle at all, but it is useful, and English is not the fastest language she's heard - and then casting the spell for the first time.

She starts as she means to go on.

Using this strategy she will successfully cast it first try, near the end of the class period, by which time Karina and Phil have both at least partly managed it via the less methodical 'keep trying until it works' strategy and the other three haven't.

Professor Weasley occasionally glances fascinatedly at Ophelia's extremely regimented process, in between troubleshooting for each of the others in turn, but doesn't interrupt her. (Most people who are like this are not quite this much so, but it's not a species he's entirely unfamiliar with; they're best left alone, in his experience, until they specifically ask for help, because unlike many kinds of prideful academics they will in fact notice when they need it.)

Ophelia will occasionally say, somewhat loudly, and then practice, something that seems oddly relevant to the struggles of her fellow classmates, not that she would be so crass as to imply they needed help.

(This is clearly not the first spell she's learned, if Professor Weasley has any clue whatsoever.)

"...Really, what I want to know is why the spell's name, roughly translated into English, means something more like 'excessively green'.  It produces red sparks; it's not even using the heraldic color - I do believe that it would be...gulemillious, to do a direct substitution - but regardless, it's just wrong.  You have perhaps 'ver-', verde, vert, verdant, or perhaps vermin's 'vermi-' if you stretch, and I would not - '-milli-', million or millipede or millimetre, and obviously '-ous', 'of or having the properties of' seen in such delightful coinings as squamous, infamous or courageous.  And what does this 'excessively green' spell produce?  Red sparks.  ...Though from a color theory standpoint, perhaps the point of the invocation is that it's in counterbalance to a situation where there's too much green around.  Not that I'd hazard a guess as to why you'd be thinking of that when you decide upon emergency signals.  If, indeed, this spell was originally designed for such a purpose, instead of pulled out of a hat when the Ministry went looking for spells -" She purses her lips, clearly having had a thought she doesn't like - "almost everyone, certainly everyone both able of body and mind - could cast, that were sufficiently impotent that even a child could cast them if they knew the trick.  ...Excuse me, even we could cast once we knew the trick.  And the worst you can do with it is spray it in someone's face to make them flinch, which, really, if you're resorting to that, I think the situation is quite beyond recovery and you should have had a knife.  Or the delightful Muggle invention of concentrated aerosolized capsaicin - better known as 'oh no why is my mouth on fire' for those of us who have ever experienced Indian curries.  I suppose it's fit for purpose in that way, though.  One does not often find oneself in a position where arming one's children with deadly weapons is a preferable tactic; a book I happened to be reading this morning quite strongly declaimed such tactics as the tools of the foolish or desperate, and I imagine that an author who lived in a time where wizards were quite present in field battles, the sort where thousands fought and died, would know."

"Ah. Yes. Well, that would be because there is a shade of red that is called 'vermillion' in English. Not having invented the language I cannot speak to why that might be."