Koibito wa 16-sai

From Chibi Manga:

Sawaguchi Ruka, is 20 years old and is a 2nd year at a technical school. During high school she was hurt by “her first love” and after that she has not been able to start a new romantic relationship. At that time, by accident she met a “Soushirou” a guy that was trying to pick her up—at the beginning she thought that he was irritating but, her “pain” disappeared and she started feeling excited and without knowing it, it turned into love. But, there was one thing she didn’t know. You’re “16” years old–

Included one-shots: K no Ouji-sama; Osawagase na Kanojo