Maririn has vanished

Kyoko Marihara, a 24-year-old teacher in Japan, has lost her memory of the past seven years due to a blow to the head, and is now a 17-year-old. 24-year-old Kyoko looks and acts like a different person from the serious, happy-go-lucky Kyoko of her 17-year-old days. She is shocked to find out that she has become the teacher she had always wanted to be, but her students are cheating on her, and she is separated from her parents, whom she loved very much.

(from mangaupdates)