I did not major in education, so I don't know what is taught, but in real life, in many social circles it is just the thing you do to give birthday "spankings". They are not abusive, but lightly done in jest. I know my own family did them (though I don't carry on the tradition for no particular reason), and ther was NEVER any innappropriate motivation behind it. ANd I would not have the least bit of apprehension if someone else had that as a tradition.
If it was offensive to somoene, however, the teacher should know, so he is aware it is not socially acceptable to everyone. However, the teacher should NOT automatically be presumed to be doing something innappropriate.