An accusation such as this, particularly in today’s climate, is obviously serious and must be investigated seriously. But your reaction suggests that your volunteers are not fully part of your team and are being treated as something akin to second-class citizens.
What would you have done if the accused had been a regular teacher? Would you have suspended the entire staff? If not, why did you remove all of the volunteers? Obviously you want to suspend the accused during the investigation, but why remove everybody else—as if they were all culpable or couldn’t be trusted—and disrupt your program entirely?
With regard to your insurance, you have promptly reported and generally cooperated in the investigation. You need to cooperate with the carrier’s investigation or you could find yourself without coverage. Your volunteer, however mortified he may be, should also report to his carrier. Failure to report could cause him to lose any coverage that he may have.
You should point that out to him that if a claim is filed you will have a conflict of interest. You will claim that you did everything possible to control the risk of a situation such as this and that if he did what was alleged, it was an unauthorized act for which he would be solely liable. Even if your insurance generally protects volunteers, that type of conduct is probably not covered or may be denied for lack of cooperation. It is much better to report and cooperate now than to face that huge financial risk at a later time. It is his choice, but if he refuses and is denied coverage under his own policy, it could also hurt you because there could be a smaller pot with which to make a settlement if a claim is actually brought.