14 Going on 30 (1988)

Danny is hopelessly in love with his teacher Peggy Noble. Of course she doesn't even notice him while she plans to marry the obnoxious coach Roy "Jack-jaw" Kelton. When Danny's friend Lloyd invents a growth accelerator for fruit and vegetables to "solve world hunger", Danny uses it on himself and as an adult tries to show her Roy's real personality. However at his first appearance at school as an adult, he's mistaken for someone else...