@sozobe,
Dang indeed.
He had told me he won the car as a prize for doing good on a quiz.
Things went great with the teacher this morning. We managed to catch her a couple of minutes before school started. Mo returned the car, made his apology, explained that he knew what he did was wrong, and promised it wouldn't happen again. Teacher thanked him for returning, apologizing, explaining and promising and said she was proud of him for admitting that he had done something wrong. He ended up feeling pretty good about the whole thing.
You know, I know it's my job as a parent to figure this stuff out, get the truth, make sure he understands what he did is wrong, and help him set it right but it is absolutely positively soul crushing to be the person who doesn't believe him. It just kills a little piece of me to not believe him when he says he won a prize for doing good on something. I want to be the person who always believes him.