I think he tricked the teacher into thinking he was a masochist but in reality he was a sadist and enjoyed completely ruining him as much as possible so he could delight in his suffering.
Yes, this was left open, but I'd guess he's a sadist too, like, just look at the way he acts and talks from the beginning!
I guess this ambiguity is also in the title. It's a reference to Marquis de Sade, from whom the term "sadist" comes from. But it's never finished, so it's Marquis de...Sade? Maso?
So...do we know what's wrong with the student? Like what beef he had w the teacher orrrr...?