I agree with everything, but I don't think that green tree frogs would make a good choice at all for this. Although they're hardy, I do think that they'd be easily stressed by a room full of kids looking at them and tapping on the glass.
Then again, people keep leos in classrooms, and they're much more easily spooked! But for the sake of the animal, I think you should stay away from the smaller, more spookable, frogs. It would also be less likely for the frog (if it's a tree frog you choose) to escape it's environment (remember--class full of kids!), and either get lost or stepped on.
Remember that the amount of money that can be spent on this and the air temperature and the humidity in the room will matter on what you get. I hope you find the best animal and that your kids are very happy about it!