Other mistake: When Cady gets back her test (the one scored with a 98) from Ms. Norbury, the date on the test is October 14th, 2003. However, that scene is set after winter break. (00:45:30)
Other mistake: When Ms Norbury passes back the failing grade on the math tests and states she needs it signed by her parents, the test states it's the "Limits Test" but the test is on convergence and divergence of infinite series.
Other mistake: When the girls are going wild, the two Vietnamese girls, who made out with Coach Carr, fight and argue. There is a subtitled translation at the bottom of the screen and one of the girls' lines is translated as "Your the slut." This should have read, "You're the slut."
Other mistake: The word Artist is spelled "Artitst" on the senior artist credit.
Other mistake: The layout of the cafeteria is different to that of Jamis' drawing. The position of several of the groups is wrong in relation to other groups. Even though the camera pans round quickly, it's easy to see that they don't match. Also, the drawing shows the "fat girls," etc. being on the back row next to a wall, but there's another row behind them.
Answer: Regina enjoys manipulating people, and she views Cady as a relatively easy person to manipulate, finding her more useful/amusing to keep in their circle. It's a running theme of the film that the friendships between all of the Plastics is very tenuous, because of the catty, manipulative nature of the girls.
Purple_Girl
Also, if Regina tried to ban Cady, people might think she was threatened by Cady. Better to keep Cady around and attack her in subtle, manipulative ways.