From Selene Scans:
Hibiki and her friends are approaching their last year in high school, and while they both have many crushes Hibiki sticks with one. In the middle of her junior year she falls in love with one of her teachers and learns that love seems more difficult for adults then for someone her age. After learning that her teacher has feelings for her as well, they start to get more into their relationship.

Side story in volume 1: Tomodachi Nante Iwanaide

{Correction from Silvermagic}