The problem is there is no such thing as a 100% success rate in this game.
Not when landing on a normal GA sized runway.
The student might do something daft like drop the gear or the flaps, and then where has your glide ratio gone?
One should simply not take risks like this. It is pointless. In the extremely rare chance of a real engine failure, most pilots who can fly can put it down into some field - IF there is a field.