Why is Brenda Song in the same grade as the Sprouse twins on the Suite Life of Zack and Cody?

Okay, for some reason I'm up late at night watching this ridiculous show with my sister and I want to know why, when the show first started, this "London Tipton" character was supposed to be way older than the twins and now she is in the same class and grade as them? Brenda Song, the actress who plays her, is five years older than the twins. She is 23 freaking years old. Are the producers of this show just thinking their audience is too stupid to notice? I don't know why I care, but it is bothering me.