They should be taught to pick an aiming point 1/3 of the way down the runway and use the flap when they are assured that they can make the aim point. At the point when they select full flap they should be able to glide the rest of the way and make the threshold. The flap is only used to bring the point of impact nearer the threshold.
At no point after passing the downwind end of the runway should the aircraft be in the position of not being able to perform a forced landing and not make the runway.
In fact if you are doing a CPL test doing the circuits and the examiner pulls the engine if you don't make the threshold its an instant fail. And I know a few that have had partials for getting outside the glide range while doing an approach. The examiner spots it, pulls the throttle they can't make it. The caa has another 400 quid.
I'm afraid I disagree. For the benefit of students looking at this thread who are confused by the varying answers this forum throws up:
There is no requirement for the aircraft to be able to reach the threshold on a normal circuit, that's why we teach, fly and test glide circuits. Nowhere in the CAA standards documents does it stipulate that in the circuit the runway must always be reachable if the engine fails, and that applies to the CPL skills test too. Please show me any such stipulation from the CAA.
There may be some confusion here with the glide circuit where the student is required to demonstrate the ability to make a glide approach to a safe landing, but that is demonstrated academically from a differently set up circuit, ie the glide circuit.
A powered approach should be flown with power applied, aiming for touchdown close after the threshold. You will not be able to make the runway if the engine fails at 100'on a powered approach to a 500m strip, in the same way you won't make the runway if it fails at 250' after take off.