|From what I gather
Spoiler (highlight to view)
they separate coz the sensei's mom fell sick and she needed the job/money and her ex-fiance places an offer that he would provide as well as not take it out on his illegitimate brother. and the boy thinks he was used and ditched by his teach (yeah, PAH!) then one ally who just happened to work under the principal but 'secretly' supported this teach-student couple informed the boy of what actually happened. Boy realizes, runs to teach's apartment but by then female teach has cleared off and maybe just maybe when she was being 'violated' by her re-appointed fiance (sorry for this unpleasant detail), she had shed a tear for the boy...well, it was a no go relationship anyway. It seems most such relationships with reversed sexes end up better in manga than if the guy is younger but yes, the age gap here was huge so barring the fact this is a taboo/illegal thing- they still had no choice ..bit bummer, eh?