Schoolgirl's Reunion (1977)

In high school, a plain-looking young girl has a crush on the school jock. He convinces her to have sex with him, then dumps her, which breaks her heart. However, years later at the class reunion, things have changed. The "plain Jane" has now blossomed into a gorgeous young woman, and the jock has turned into pretty much of a klutzy goof. Complications ensue.