Hi.Me.Go.To (2001)

Tomomi and Takashi are two high school students who are trying to take that next step towards adulthood - losing their virginity. But Takashi's lack of passion really puts a damper on things until he accidentally spies his teacher, Misaki-sensei, getting felt up on the train one day. He decides he wants to give it a try, and soon after, he and his teacher have an affair.