This actually sounds really interesting. I unfortunately don’t know the name, but I would like to know if someone else does. For some reason while reading your description, it honestly made me think of Shotgun Boy. Not the same plot but similar I guess?
I think I finally found it, I searched myself through the deep nooks and crannies of this site, it was "Futsuu to Bakemono",and to add on the mc thinks the monster classmate is cool.
Oh, nice! Thank you for telling me <3
It was definitely a manga in black and white, the vague plot is basically this school girl(mc) find out her classmate not human (classmate was either a class president or very popular at school) she accidentally sees her classmate eating/killing somebody (was most likely their homeroom teacher, I just remeber it was an adult man).......the classmate takes the shape of this alien thing (Like Venomn from Marvel), i don't remeber what happens after that but it seems classmate is contemplating wether to kill mc or not,i'm not sure if mc knows that the classmate is aware that she knows(they're both girls, and it has this sort of shoujo-ai vibe to but also thriller)