I find students who are having difficulty judging flare height are often a bit overwhelmed with all the things happening in the final stages of landing.
One exercise I have had some success with is to fly a good approach and flare but just before the wheels touch I add just enough power to keep the aircraft airborne in the landing attitude. I then deliberately balloon slightly with a bit more power and then let the aircraft sink back to just above the runway.
The student has nothing to do but watch the sight picture. I get them to tell me how high they think we are and whether the aircraft is going up, down or steady.
I found this let the student calibrate their eyes and without the pressure of flying the aircraft lets them process what they are seeing.
Finally excessive speed makes landings harder for new students because it unnecessarily prolongs the flare which gives the student more time to screw up. As was noted the POH speeds are for gross weight. A C 172 or Pa 28 with 2 students and half tanks will be at least 300 lbs below gross weight. Reducing the approach speed by 5 kts works wonders for all of the typical trainers.