https://www.dlsite.com/books/work/=/product_id/BJ01305102.html
As always, please support the artist and buy his works.

Netorare Childhood Friend - A simple, virgin girlfriend is studying sex with a flirtatious guy.

Emi and her childhood friend Ma-kun are always together and think they will get married in the future.
As a girl of her age, she is interested in sex as much as anyone else, but Mar-kun, a serious man, does not take her seriously until she gets married.
Then she meets Jun Shimada at a prep school she went to on her parents' advice. Jun, who asked to teach him how to study, has a flirtatious appearance and cheerful personality that is the opposite of her boyfriend's. At first, she is intimidated by his appearance.
At first, Emi is intimidated by Jun's appearance, but they soon become friendly.
Jun tells her, "In return, I'll teach you a naughty lesson"....