I think it was Jeff Foxworthy who said (when in trouble with your wife, but equally applicable here): "It's just like talking to the cops. Stare straight ahead. Only speak when spoken to. No sense copping to things they don't know about yet. I Say as little as possible until I find out what exactly I'm being accused of."
EDIT: Being called/sent to the principal's office isn't always about being in trouble. In high school, I was called to the office for lots of different things: to talk to police about someone who knew me and her dealing of drugs, to address a change in my class schedules, to get the message that a member of my family died, to be given information about volunteer work and various extracurricular projects, to receive a phone call (seriously, just to answer the phone once)... all kinds of stuff.