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.
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)
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.