The Big Bet (1986) on Netflix

Plot summary

Seventeen and still in high school, Chris thinks about sex more than anything else. Fantasies about the women he knows fill his days, and chances that once came easily are gone. Then Beth arrives in town, and Chris makes a bet with the other guys: he will get her into bed within ten days. His chief rival has a way to stop it, threatening to reveal that Beth was accused of taking part in an orgy at the school she attended before, a charge that was false, if she sleeps with Chris. Unsure what to do, Chris turns to Michelle, the older woman who lives across the street, for advice.