Could we go to jail?

I rather doubt that you'll be bounced into the clink for chucking a few rocks that didn't damage anything. Even if the police managed to ID you, they'd most likely just have a little chit-chat with your parents. Personally, I think you should find the courage to tell your parents what happened. They'd likely be proud that you had the courage to confess that you were present when it happened. Then, if a cop does show up, he'll meet two very proud parents who will inform him that they are well aware of what happened and that it won't happen again. The cop will likely also be shocked, but pleased, that you had the courage to step forward. Good luck to you.