I fear that you must. Although the school is well within its rights in confiscating prohibited items, it has a duty to take
reasonable care of them and to return them in due course.
In the matter of value, you are, to some extent, ovr a barrel. However, the parent should
provide a good description of the missing items and obtain a
valuation from that description from a reputable jeweller.
The loss may be covered by the parents' home insurance policy, in which case the school might settle for meeting any excess on the claim. Otherwise, you will have to pay up and tighten up your procedures.