Brakedwell
One solution is to teach both techniques are appropriate depending on the circumstances. So for the ab initio phase keep it simple. Aiming point technique etc and don’t waste useable runway behind you on landing. Emphasise that if landing on a limiting runway care is needed eg excessive approach speed, longer than normal float.
Of course the student needs instruction on how to use and interpret landing data. Is the data for the ground roll or assumes 50 ft above the threshold? Those are the two I’m familiar with perhaps there are others. Always happy to be informed, corrected etc.
For an aspiring CPL holder explain the techniques employed on a jet transport may, or may not, be the same. In that context explain that landing “on the numbers” is not correct in this type of operation but it might be necessary flying a light aircraft onto a short runway. In particular I would suggest any “ducking under” will likely lead to swift admonishment from the trainer! In that context I always understood that if PAPIs are available then they should be used. I don’t recall an official height below which they can’t/shouldn’t be used but as Olster said 200 feet seems reasonable. Of course that generally corresponds to Cat 1 decision height. Anyway, hope that helps. Interesting discussion.
ps to further confuse matters some approach plates tell that you that the visual guidance and instrument procedure don’t coincide.