Looking for a Japanese song

I remember hearing one song on YouTube before that I really liked but I can't remember what it is. It had a whole animated MV, and I think Hatsune Miku also sang it but I'm not sure. The song is about a girl who's getting abused/bullied/something like that, and a boy at school decides to come and help her. She ends up venting to him and crying. I remember one of the lines being something like "How did you learn to fake a smile so well" (probably not exactly that, but gets the point across). Does anyone have an idea of what it could be?