They were all whipped and lost a lot of house points and got many detentions! Also are they not at all concerned about how they are now wronging Karen, an unrelated bystander.

They haven't hurt her! And the merpeople don't appreciate hearing afterwards "oh it was sufficient", that's not the humans' decision to make.

But keeping her here for a long time is kind of terrible and her charm won't last forever what if they take a while to negotiate or something.

They'll let her refresh it.

So they're going to let her have her wand?

They'll give it back when she needs to refresh her Bubblehead charm.

She's cold.

The one who speaks English has left.

She's really cold maybe she can mime it.

Maybe it works because a seaweed sort of blanket is offered.

Does it, like, work.

Helps a little.

She drapes it around herself but mimes more cold.

Shrug. 

Minor is trying to get permission to go talk to them since he speaks their language and they have kidnapped Karen.

They are emphatically not allowed to have Karen! Who is trying to prevent him from talking to them?

"Then they'll have two hostages," says Professor Mole irritably. "Both of you go back to your dormitories, we're working on getting it sorted."

"What if they don't know how to keep a human underwater this long she could die!"

"I assure you we will not permit that to happen, Swan, now go."

"How are you not permitting that to happen."

"We've notified Aurors who are on their way in case the merfolk cannot be swiftly persuaded to see reason. Ten points from Slytherin for not desisting, I am busy."

Is she busy with anything Karen-related.

Yes, she's talking with the Ministry and ordering students out of the way and professors down to the lake.

Fine, Miranda will leave her alone.

Hogwarts is not handing any students over and demands the safe return of this one, lest they conclude it is not safe for the student wizarding population for there to be merpeople in this lake.

Merpeople have no objection if Hogwarts cares to relocate.

It is clarified that that is not the means by which they intended to achieve a school grounds devoid of merpeople.