I don't think I've ever suggested that standing in the undershoot is a sensible thing to do. The chances of being "collected" and the distraction you cause are all reasons for not being there. But we entered this thread with a post from someone who was unsure if they had done anything wrong and none of us are any wiser because of the poor attitude of the instructor. And please, let's not have anymore ridiculous scenarios regarding student pilots. The undershoot of any runway is generally not the best place to be.
And while we are here, only with one or two exceptions have I ever seen a glider airbrakes shut that didn't have enough energy to miss a person, even a nasty glider like an Astir can be made to hop over person. The exceptions were gliders in competition finishes which had been "luckily" judged to the inch. The problems with early solo standard pupils, excluding poor SA, are generally due to having far too much energy or a poor appreciation of the performance and control they actually have.
Would you want to fly with someone who you felt had endangered both you and himself and then became all self-righteous and stroppy with you?
Who said anything about anybody being self-righteous and stroppy? Someone is making things up. And yes, I'll happily flying with anyone, even people who really annoy me but especially those who need instruction. Because that's what instructors do. What they shouldn't do is go flying when they are having problems at home.
Which brings me on to a larger point. Gliding clubs often have drives for new members and most spend an inordinate amount of time and resource in marketing themselves. All of this effort is wasted if the new members are chased off by childish instructors - which I have seen far too often.
My comment regarding competition finishes was made so that a pointless death like that of Neil Lawson could be prevented without detracting from the sport. Safe flying is all about risk management but a total prohibition on certain manoeuvres and aspects of gliding will do nothing to improve overall safety.
PM