My son came home with banded mystery snails

So... As the title says, my son came home with some accidental pets from a lake trip he took with his school. He grabbed the shells thinking they were pretty and didn't see anything soft on the bottom so he pocketed them and brought them home. Apparently these snails create a trap door at the bottom to seal in moisture so it's hard on the bottom, but they are still alive. I put them in a bowl of just regular tap water and I keep finding little sand deposits so they're definitely removing waste, but I'm not seeing them move. They're invasive so I can't flush them and I don't want to kill them. My son wants to keep them as pets so can someone tell me what I should do? I want to be a good snail owner and I want this to be a positive experience for my son!