actually, i understood it as "continue keeping it a secret relationship until you graduate." moreever, they even try not to be seen walking together in school to keep it like that and they both agreed on that setup too.
it's obvious that the uke only likes the brother as a little brother but not more than that and sensei genuinely likes the uke.
As in the first chapter of this story when they are in the infirmary the translation gets kind of weird. I just assumed that the sensei actually meant to keep the relationship a secret until he graduates. Besides, he says he has been in love with Aikawa two years prior and they have only a year left. If it were just a momentary fling, I don't think he would have chased after Aikawa for two years.
To be honest, as long as this didn't finished in a threesome (I got a scare around the second part) I was fine with either. Don't get me wrong. I like a good threesome. But with personalities and behaviours set in part one, a threesome would have felt out of place.
...the phrase the sensei in the second story kept repeating? "We'll continue this relationship until you graduate." That phrase tells me Aikawa is yesterdays news come graduation day and Sensei will move on to a new "student target". Aikawa should have chosen Shin. Granted, they'd have double the stigma (gay & incest) but who cares!! It's no one's business if two consenting adults sleep together. (When they become adults. LOL)