I think a mouse or rat would be ideal, this is usually what our schools have, however I have often been a bit concerned about the care of School pets, IMO it has not been of a high standard in our schools, children volunteer to take the pets home on school holidays here, who knows how good they are at taking care of them, and too many of them have died.

I know you will take good care of the Pet, and I wish you well, its good to beable to teach children to be responsible at a young age for animals, some donot have that experience , so through the school they can, GOOD LUCK!!