i was in the same spot as your kid earlier this year, except i had the $200 cause my sister needed it before me, and some little punk stole it and claimed he"found" it...
so i approached him about it, he says theres no evidence its mine. i ask for it back, cause we all knew it was mine. so i walk away.
oh, and by the way, im a very physical threatening person, being 6'4" and broad in grade 10 isnt always the greatest match
luckily im physically threatening, and i wakl up to him the next day and politley ask for the calculator back, he says no. i say then we can go behind the school and discuss this after school is over... he gave the calculator back..
two things i learned from this.. the teachers dont care, and the kids my age are kinda wimps..