In a class by themselves

Harry Potter and the Order of the Phoenix: Chapter 16

You have to wonder what kind of trouble having an underground class is going to get these kids in to. I wonder if Umbridge's classroom is big enough to keep them all in detention at the same time. But maybe they will be able to keep it a secret from her royal highness The High Inquisitor.

You can't blame them for wanting  to learn how to defend themselves against dark arts. Knowing that V-Voldemort is out there and not knowing how to defend yourself is a vulnerable feeling, so I say good for them. What is tyranny if not something to fight against. One side will call it treason, the other will call it self-defense.

Give Hermione credit for knowing how to run a meeting. First, she picked an out-of-the-way place off of Umbridge's turf, which every good spy knows to do. Then, as soon as others got them off topic, she brought them back to the question of when and where to hold the class. They ought to meet at Hagrid's place. Umbridge won't have any surveillance cameras there. And Harry should break out that map that GPS tracks everybody during class time so they will know where Umbridge is at all times.

What they must be wary of is a snitch. Keeping the Slytherins away would be a good start.