I don't remember much but it's kinda still a newish series, the characters are in high school and one day the girl is in the library or a closed room and then suddenly a guy and girl walk in and start having sex the girl who was already there just kept quiet until it was over then afterward she asks the guy to teach about kissing and sex

The girl is kinda monochrome and is curious and doesn't understand certain things like relationships or something like that

Found it, it's called

GAME -in High School-

sounds interesting.

Okay this old but I'm curious anyone knows any similar shoujo like the one I mention here, I'm dying to find something to read as this series has barely updated