You shouldn't break promises.
Joey tells Timmy that she'll go to the prom with him but then the star quarterback at the school asks her to go with him instead.
Joey wants to have a great time at the prom.
Joey tells the quarterback that she already accepted Timmy's invitation to go to the prom and that she always has fun with Timmy.
Joey tells Timmy that she can't go to the prom with him and that she'd rather go with the quarterback.
Timmy has the night of his life at the prom with Joey.
Timmy is crushed and instead of going to the prom, he stays home with his grandmother.