Visible crew/equipment: Before Cady enters the house, when one of her best friends is watching a movie the reflection of a crew member is visible on the TV.
Visible crew/equipment: When Damien and Janis drive off leaving Cady on the sidewalk with a picture, a large square stage light on the set is reflected in the window of the back car door (probably one which lights the ground to give the effect of a streetlight). It is too large and bright to be one of the lights from a house across the street.
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.