Well, if you ARE going to bring it to school, keep it in your locker (make sure you have a lock too) instead of bringing it around with you. That way you won't lose it and it won't get stolen.
I always follow my strict policy (above ^) even though sometimes I want to use my phone in class, I care more about my money. :P The only thing I've ever lost is a clarinet ligiture.
Lucky you that that kid found it. :)