Tsubomi no Hokorobi

Yuina, whose parents are divorced, secretly visits the home of her stepfather, whom she has feelings for, without her mother knowing. Though he pressures her into a sexual relationship, she refuses to go all the way.
One day, just before summer vacation, her friend Koharu confides that she plans to have sex for the first time with her cram school teacher during the break.

[Translated from dlsite]