In one of my stories my mc did something bad when he was a teenager and it ended up affecting him badly throughout his life. When he was 17 he and his brother were in a foster home and the foster parents were abusive. So he burned down their house while they were still inside. They got out all right. The only thing is I'm not sure what he would be dealing with in terms of the law.
The statute of limitations varies from state to state. Check it for the state your story places the foster home in. For Minnesota it is five years (unless someone dies in the fire, in which case there is no limit). Google is your friend.
He'd face an arson charge if the authorities learned he set it on purpose. Depending on the circumstances, attempted murder might be on the table too.
The fact that's he's under 18 would make a huge difference. Depending on the judge and on how the prosecutors wanted to try him, he might be sent to juvie or boot camp instead of prison/jail.