I agree with Plonis. I think that there are lines that shouldn’t crossed, even to show positive affection. But not all affection should be abolished.
Though, I still have a hard time believing that most Rebbes nowadays would even wanna show affection. I had a rebbi that was somehow LOVED (by many kids), who would commonly grip and pull my ears for not paying attention. My friend even started bleeding once.
Smacks by rebbeim were done out of anger, and were common too.