Just quit. Tell them you can no longer work on the project, whether you want to go into detail about why is your own decision. If I were you I would also ask them not to use the work you already did for them, or to pay for the rights to use it, or at the very LEAST give you credit for it.

Doing work on spec is a bad idea in general, for a LOT of reasons, and really the fact you are working with high schoolers who have no professional experience should have been your first red flag. There's a tiny fraction of a chance that they could finish and get lucky and make a ton of money, but I can tell you from personal experience that projects like this tend to end in two ways: They don't get finish, or no one wants to buy them.