It would have to be something that can accept living in a cage. Rats are smart. Could your class enjoy a white rat or are rats too unappealing? A friend who taught at a girl’s school used to bring his bird in. Birds could be fun and a parrot or mynah bird even more so. And parrots live a really long time. Hamsters are really dumb. I don’t think they would be fun. Cute little geckos, newts, and turtles could be interesting. Turtles can live a while and can be handled by children. A red-eared slider turtle, a common pet, can live 40 years or more if they receive good care.