The above. STRONGLY.
Mum used to teach kids to ride, on a bombproof pony, on lead, in a round pen. Because of all the precautions she took, her insurance was relatively cheap, but they MADE her make all her clients sign a waiver, or she would not be covered. And that was with a comprehensive safety plan, with proper safety gear (helmets, riding boots, toe-stopper stirrups, one kid had half-chaps that her parents bought her), and the kids weren't even allowed near the pony without their helmets on.
To have a horse rearing on command with a rider on board who cannot see properly is much more dangerous than that. YOU MUST HAVE INSURANCE, and any good insurance company will insist that the stuntman sign a waiver.
Don't come crying to us when there's an accident and the stuntman sues you for every cent you're worth. We did warn you.
Edit; also, our insurance, for 1 year, was just under $2k, and covered us for anywhere on the planet, for up to ten million dollars. We had it VERY cheap because of the safety measures we took and the one-on-one lessons, a larger or riskier operation would cost more. But it IS WORTH EVERY CENT.