So you would get £35/hr, but have to cover all the fixed costs, hangarage, insurance etc.
Doesn't sound like a very good deal for you to be honest. It would mean the other 40-50 quid per hour goes to the school. Hmmm, I think you need to renegotiate the rates a bit there.
An example, we pay £20ish/hr for a 150, but we are responsible for parking and Insurance. Good for both us and the owner. Your deal sounds a bit one sided.