Pretty sure it was the teacher.
Knowing that it's already two years later where they got to live with each other, i'm confident that it was from the seme.
Actually that story is a flashback, it meant two years later after finding Bunya-san, which happened when the kids where 15 (as it shows the small text balloon below Tai-chan), so those hickeys are the ones the teacher made, the author is trying to point at the time where the relationship between the protagonists starts (aka the start of the manga).
thank you :3
Who made those hickey? http://www.mangago.zone/read-manga/boku_to_neko_to_hatsukoi_kousa/mf/253896/Vol1/pg-8/ is it the seme or the teacher or the worst...uke's dad?