So, I'm looking for a yaoi about a senpai / kouhai relationship taking place in university environment, with som high school flashbacks of how the uke met the seme. I remember that the mangaka drew the uke to look almost malnourished. You could see his rib cage. The seme is borderline rude and at one point asks the uke if they should have sex in front of his friends, because his friends were joking(?) about their relationship or something.

Does this ring any bells?

I found it myself. 🤣
It was Color Chart wo Kimi ni for those interested in checking it out.