Moral of the story: Never let your parents leave anything in your car they may later need
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That sucks, but your mother probably didn't know what to think when she was told you weren't at school. What you should have done was to call home and let your parents know you were having car trouble and so they could let the school know you'd be late. Although this sounds like a minor offense, a parent's way of thinking is "If I can't trust him/her with little things, how can I can trust him/her with bigger things?" Trust me, I've been there done that, thought my parents were the uncoolest people in the world for it, but later in life you will see why they reacted the way they did.