She spends the next little bit on discussing Hogwarts' rules about duels and other fights between students - you need a Professor's supervision, either in class or in a related club. Hogwarts, like the Ministry of Magic, does have a concept of self defense, but the bounds of that are pretty strict, and it's common for both parties to a fight to get in some amount of trouble.
She also discusses spell safety, a bit more in depth than their Orientation class, explaining the why of it all - many offensive spells are similar, and if you don't keep a clear head, you can easily overload or alter a spell, risking harming yourself, your opponent (more than you intended to), and any bystanders. (She gives examples of cases she's seen, not flinching away from how some of these are rather gruesome.) Offensive spells are also often aimed over a greater distance than other spells, usually at a moving target, and if you miss your spell could end up harming bystanders (common in school fights) or could damage your environment somehow. (She knows someone who set his house on fire while Aurors were trying to arrest him. The Aurors got him out, but he had some pretty bad burns.)
Because of that, you should avoid using spells you are not intimately familiar with in a fight, unless you have absolutely no other choice. Avoid seriously harmful spells if bystanders are around, sticking to effectively harmless spells if there's an outright crowd. (Many spells are designed for harmlessly subduing your opponent - they have risks, like all spells do, but those are lower.) Keep your environment in mind - don't try to set your opponent on fire in a wooden house.
And as promised half the class actually ends up dedicated to stretching, basic exercises, and some practice catching themselves in a fall. Most of the Slytherins grumble about this.
Professor Reynolds mostly just seems amused. And, after assigning the week's homework - a chapter in Duel or Defense, and one in Arm Yourself - she says, "This Saturday, since most clubs won't be meeting yet, Professor Flitwick and I will be putting on a dueling demonstration for students. First years are required to attend, unless you get permission otherwise - we'll be working through some of the basics I want you all to know." And a small smile. "And, yes, there will be cool spells used creatively."