It's all a prison, though this is maybe a bit smaller than most are used to. I just think it was a bit... overly optimistic for them to think they could escape to some perfect safe haven with no strings attached. Be careful what you wish for and all that.
It always seems to be a sort of add-on to a general science class. I'm not sure it actually teaches the children to care for the pets, since the teacher probably picks up the slack so the pet doesn't die of neglect.