Hi,

Right straight: There is this manga where a Female_Lead and her somewhat called best friend; the oh so called best friend's boyfriend cheats on his girlfriend, and the F_L witnesses it (him kissing a long-haired girl behind curtains inside class) but hesitates to tell her friend only to get framed that she is whom he cheats with and the oh so called best friend chooses to not believe her but the others (her boyfriend or the long-haired girl and/or the public schoolmates?) yeah, it is a very cringey start. And oh yeah, I'm sure the romance is revolving around female student and a male teacher.

I'm looking for that.

Thanks for reading in advance.

Ah! I've fount it.
Sore wa marude Amagasa no you ni

There.