A Fairly Odd Movie: Grow Up, Timmy Turner! (2011)

The television movie is set in the city of Dimmsdale and centers on the series' main protagonist Timmy Turner with his fairy godparents Cosmo and Wanda and his fairy godbrother Poof. In the movie, Timmy is now 23 years old but is still in fifth grade with his fairy-obsessed fifth grade teacher Mr. Crocker. Despite being grown up, Timmy finds a loophole in the fairy rulebook Da Rules: if he continues to act like a kid, he will still get to keep his fairies. However, the dilemma rises when Tootie, who was once a dorky girl when she was 10 years old, returns to Dimmsdale as an attractive woman. Timmy falls in love with her, a sign that he is growing up to an adult, which means he is closer to losing his fairies. Meanwhile, an oil business tycoon named Hugh J. Magnate, Jr., who teams up with Mr. Crocker, plans to use Timmy's fairies' magic in order to promote his oil business.

Source: TMDb