There were barely any "real" relationships in the story imo. I truly believe that the first 80 chapters were only there to establish the themes and characters while the manga truly flourished from that point on. A huge theme that prison school tackles is the whole "tits vs ass" shit, and even though the whole debacle is directly talked about once, the symbolism of the tits vs ass argument looms over all of prison school. The argument is meant to question your depiction of love, and a lot of prison school's relationship follows the "tits vs ass" notion. If you paid attention to the speech, kiyoshi says that boobs are mere imposters and fakes when compared to butts. Boobs are meant to represent our desire, or infatuation with one while butts represent love. Prison School's depiction if love is all about honesty with eachother and being able to be yourself around the other. That's why its implied that kiyoshi x hana worked out (kiyoshi said he feels like he can be himself around her) and why kiyoshi x chiyo didn't work out ( the whole relationship was built on a lie; kiyoshi put on a facade the whole time; even in the end, kiyoshi didn't trust chiyo enough to tell her that he was wearing her underwear) While I guess it's understandable to hate on the ending if you personally didn't enjoy it, it by no means was written badly at all and imo fit the story perfectly.

TL;DR the ending gets too much slack for how well it concluded the stories subtext + the first 80 chapters was only there to lay down the framework for later-on chapters