Thanks to every one who has taken the time to reply.
just to clearify two points.
1) If the student incorrectly raised the collective during a simulated engine failure, this would reduce RRPM, and could lead to excessive flapping and the tail boom coud be chopped off. Whould the rotors stay attached and tulip?
2) Low RRPM (but the blades not stall) can lead to excessive flapping?