There was this one yaoi manga I read a while ago about a human guy who was in an all demon school. Eventually one demon found out about him, and they slowly became closer. The two made an arrangement that the demon could "eat" the guy (in a s*xual way) whenever he wanted to if he didn't want the humans secret to be exposed. Some small details I remember are that the demons could hide their horns (that's how the guy got away with being human), and that the bigger their horns the more powerful they were. Also, their horns were very sensitive. There was this one scene where the guy brought the demon to the nurse because he had passed out. His horns were out at the time and out of curiosity he touched them. The demon however felt this and later called out the guy for it. Thank you so much if anybody finds the name in advance! 🙂

Could it be Tabetemo Oishikuarimasen ?
https://www.mangaupdates.com/series/y7nh3eg/tabetemo-oishikuarimasen

It is!! Thank you so much 😀