If your friend is paying you for the use of your ménage you may require a commercial policy so it is worth checking this with your insurance company. If your friend has a valid instructor’s policy it will cover them for anything for which they could be liable. However if there was a problem due to the surface of the school or a faulty gate or something along those lines there could be a claim made against you. Therefore you should ensure that you have cover in place to cover you for any eventuality.