You may be liable for the damage, because it was negligent or careless of you to let students use a difficult to use weapon near other people's property (or other people)--the likelihood of some student misfiring and hitting a car, a window, another student, etc. with a rock was high enough that having them do this was likely negligent. If you are liable, they have the right to get the reasonable cost of the repair from you (and you have the right to make them prove the reasonable cost, such as with estimates or proposals); they do not have to go through their insurer for the damage (the law never requires someone to submit an insurance claim) and they do not have to let you personally make the repair. If you do not pay the reasonable cost to repair, they could sue you with a high probability of winning.