RTN.
If you have seen your students land long at a non vasi airport it is a lack of fundamentals in teaching the art of flying.
Just to be clear, it was never my students. I was the one in the landrover going to help recover aircraft from the ploughed field where they had over run a 800m runway. Without fail each one had been taught at a very large commercial airport, by instructors wearing too many stripes who always taught to fly long shallow approaches that simply didn't work at our simple grass strip.