Kids: Mom, how did you and Dad end up together?
Her: You see, your Dad forced himself on me, I turned the tables on him and twenty years later we are here...
Hmm...Well, I suppose it's entirely possible she left it there on purpose to trap poor Sensei with some good old fashioned blackmail.
I mean, he eventually snapped and just fucked her, but after realising it was her first time and he was hurting her, he returned to being a kindly person...Until it eventually dawned on him what he did and, well, he now has to keep this relationship going in order to keep it all secret.
Doesn't sound too bad, I mean, he gets sex, she gets sex, I'd call it a win-win except his job as a teacher might not be so safe anymore.