Non Owned Insurance for a school bus

I work as a school bus driver for a private school. The bus is owned by the school. I was asked by a sports group if I would be able to transport them to and from sports every Sunday, using the school bus I use every week. The school administrator is willing to rent me the bus. However the insurance the bus has now excludes coverage if the bus is rented out. The company was contacted to see if they would be willing to endorse the policy. The company refused even for a higher premium. My question is, is there anything that I can do to get coverage for an affordable price? Can I get a non owned insurance for a bus? Can the school get a supplemental insurance from a different carrier? Any other options?