I get where you're coming from, in real life, blackmailing someone like this would be a pretty shitty thing to do, and would be a much more serious situation than it's being shown here. However, Ayaka is basically getting an easy route to ANY college she desires, by simply getting a student to come to school and having her not fail, while others are gonna have to bust their asses off for a good 3 years to do the same. So idk man, maybe it's because i've seen much worse blackmails on here, but it's kind of an eye for an eye situation.
That is the blackmail between the teacher and Ayaka (which is just as awful, but at least the teacher isn't a reccurent character) and it's not like she get it for lazing around, she's established as the top student and it would be pretty natural for her to get the recommendation without the blackmail.
The blackmail between Ayaka and Honda is that as long as she follow one request per day, she kept the fact that she got blackmailed for said recommandation letter for herself and go along with it. She basically blackmail her by using the action she got blackmailed into doing, now that I think about it.
Dunno, that's just my point of view, but if you put the people you like into such situation, I'm not sure what that say about you.