I agree with what everyone else said, and I'll say it again, its a great idea as long as everyone treats it with respect and knows the proper care.
Also, you must have a back-up plan incase the head of your school suddenly declares a "no-class room pet" rule (it happened at my cousin's school). You also need to make sure that the animal has a home to go to during the summer and such.
Our teachers at our school are too lame to have classroom pets. We had one in science last year. A goldfish. But everyone would shake the tank and poke at it and such. As a result, the fish died.
I wouldn't really suggest anything such as a rabbit or any other larger furry animal. I think you all you try something like a tarantula, lizard, scorpion, or other non-everyday-house-pet. In other words, something that no one in your class really has as a pet so that they can learn something from it.
I hope you all decide on a good choice!