Althougt normally I do not agree with the mentality of most naive MCs that let their annoying enemies live for no real reason other than simply don't kill them, in this case I can get somewhat around this. The reason is pretty simple: MC doesn't consider, in the least, that teacher as an enemy, even more for me he just think of him as no more than a annoying bug that he could kill at any time no matter what he could try to pull of, and even if he tried to do something against his "loved ones", Lilith still is her more trusted ally and someone who that teacher could do nothing against either, and could also help MC keep him at bay.
TL;DR: MC doesn't actually consider that teacher an enemy, just a pretty annoying bug.