Ummmm, "unethical" and "unprofessional" are the terms I'd favor in describing Miss Mary's 'educational approach.'
It strikes me as pretty obvious that the woman was a 'toppish' spankophile who enjoyed paddling her students, and she was in effect 'bribing' them with the 'currency' of unearned high grades, in order to get them to accept her spankings.
While I agree that it would be a fun fantasy to write about, especially from the perspective of a highly 'bottomish' spankophile pupil of hers ("Bare-bottomed after school for an 'excellent' in deportment, Miss Mary? Okay, but I'll require thirty extra-hard paddywhacks..."), but it's despicable behavior in a RL situation.
I'd have RL issues with a spankophile teacher administering corporal punishment, even if it were allowed by the school and deserved by its recipient...

--C.K.