The short answer is no - schools do not insure pupils' property. However if you can show that the school is legally liable for a loss (e.g. if an item was given into their safe keeping and they just lost it) then they will have liability insurance to cover that. It would be the Local Education Authority you would claim against. You would however have to prove (a) that the school staff formally accepted the item into their care and (b) that the loss results from negligence on their part. If stolen during a break in there would usually be no negligence.
Best to arrange cover yourself - your household insurers may be able to extend cover under your policy.