I want to make a few points:
WIY: great post
Regarding the OP: I do NOT think its ok for a rebbe to show affection with touch. Certainly nothing more then a pat on the shoulder.
As someone who was taken under many teacher’s wings, as well as had regular meetings with social workers and Rabbis during my HS years, I was NEVER touched by any of them, not even a pat on the shoulder. And I was showed a lot of affection.
Instead they were affectionate through their own actions. I believe this is the true way a rebbe or adult can show affection. Touch is superficial. Actions go a lot further. I had one teacher that invited me to eat supper in her home with her family whenever I wanted. One Rabbi who gave me a tehilim with a personalized letter inside. These are actions that I can remember vividly today, and see the impact it had on me. Not a hug or a pat.
Now as for abuse, I strongly believe there is never a reason or excuse for it. I do not believe it makes for better chinuch or discipline.
Just because many great talmidim that were hit turned out alright, does not make it okay! You should look it as “they turned out alright even though they were hit” instead of ” they turned out alright when they were hit, so hitting is okay”
For every child that will not be long term affected by abuse, there is another one whose life will be ruined. And no rebbe or adult can decide which kids can take it and which cannot.