Are the people taking lessons riding their own horses that are boarded at your barn or are they riding your horses?
I ask this because last year I had a major accident at the barn where I board and ended up in the hospital, big bills, etc. Since the ER listed it as a riding accident my insurance company, unknown to me until they called, decided to try to get the homeowner's of the place where I board to pay for the accident and make them liable because it happened on their property. When I started boarding there I signed all of the contracts and waivers and she even has multiple state provided signs about rider assuming the risk, but when it came down to it, the insurance company still tried. When I talked to my insurance company, because I was very upset and didn't want the family where I board to be liable, they said that since I was riding a horse that I own, even if it is boarded somewhere else, I am the responsible party and my insurance has to pay all the bills. I had to send proof that it was my horse, but the homeowners were not liable. However, they told me that if it is a horse owned by the barn or someone riding another boarders horse, then the owners would have been liable. Don't know if this helps at all.
Do the people own a trailer? Maybe they should just haul to the trainer's facility.