The Teacher(1974)

She corrupted the youthful morality of an entire school! Her best lessons were taught after class!

18-year-old Sean's first summer after completing high school is much spent with 28-year-old teacher Diane, who's husband is too often motorcycle-racing instead of with her. Wacko Ralph also has "the hots" for Diane; and it doesn't help that Sean was with Ralph's younger brother, Lou, when Lou died