Responses
I think it's the Gii and Haru pair from http://www.mangago.me/read-manga/ai_to_yokubou_wa_gakuen_de/
a helpful user put up which ones are their chapters: http://www.mangago.me/home/mangatopic/1321340/
Omg thank you so much! :D
I read it a long time ago, and I think it might've been a side plot rather than a main plot, but basically the seme (at school) will randomly collar a guy, and that guy will be his slave(?) for a certain period of time. The uke (who's super hot headed) somehow got chosen this time around. I don't really remember the rest of the plotline, just that there was some bdsm involved XD