Continuity mistake: In the 'Jingle Bell Rock' scene, right after Gretchen kicks the stereo off the stage, you see her feet, and they are both symmetrical next to each other, then when you see her again one of her feet is behind the other. Gretchen does not move at all between shots.
Audio problem: When the Spring Fling Queen nominations are being announced applause can be heard for both Janis and Cady but no one in the class is clapping.
Continuity mistake: When Regina is talking to Cady about Janis, Cady is wearing a light blue ribbon in her hair. As the scenes cut in and out of Cady the ribbon changes from front to back a few times.
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.
Revealing mistake: When Aaron and Cady first speak and you can only see the back of Cady, you can tell it's not Lindsey Lohan because it is a messy, red wig and again after Aaron asks her for a pencil, you can still tell it's not Lindsey Lohan.
Answer: Precisely because he was sleeping with a student. Getting her pregnant would bring their affair out into the open and create a scandal for the whole nation to know what he was up to. Therefore safe sex was really important to him.
The_Iceman
True, but since he was spending time with her, he could make sure that birth control was being used (i.e. him using a condom). I think he was trying to appear "uptight" about sex, so he could be the least likely suspect.