Good points above. One of the hardest things for an instructor is to let mistakes just happen. This was one of the more difficult behaviors for me to over-come. I know the more veterened I got, the less I cared if a student banged one in(you know, the borderline landing that might or might not have set the ELT off). Things like those are hard for us as instructors to let go, but in the end, you have to so the student can see the "Worst Case" scenario and hopefully the next time be able to recognize the sypmtoms and recover before they set it up again.

Now if they continue to make these major mistakes over and over, and it's becoming a safety issue, have another instructor go with them and see if there's something you are missing. Using another set of eyes/ears/brains is one of the best tools out there that are definately under-utilized. As long as you got 2 instructors, have an even switch so each person doesn't feel as if they are getting shorted also.