Feign complete, frustrating ignorance. Require the person doing the talking to do as much work as possible, and frustrate them with questions every step of the way. Most of all, VOLUNTEER NOTHING! You're being called into the office not because they have proof, but because they want proof, otherwise your parents would be there and this would just be a formality preceeding your punishment.
Acting completely clueless about the affair and saying, "I don't understand", and "What does this have to do with me?" a lot really makes them work for it and lay all their cards on the table. Then you know how much they know and can construct a scenario that fits the facts they have and shows you as innocent, or at worst less guilty.
Worst case the evidence is damning, in which case you finally, say, "OOoohhhh, now I see", or "Thank you for explaining, I honestly wasn't sure what you were getting at" (because they'll never approach this sort of thing directly on the off chance that they're wrong), and then lie like a trooper until they're not sure what day of the week it is. The more totally nonsensical the lies are the better... although claiming you're actually an armadillo might be going a little far... to start with at least, maybe after you've claimed you're actually a Jehova's Witness and are in the Witness Protection Programme you can move onto armadillo status
