Depends on the parents really, some don't see it as such a bad thing while to others its the "YOU'VE BEEN SMOKING MARIJUANA!!!!!!!!"

In my situation my mother had claimed to smell pot, and summoned my father to confirm "It smells like someones been smoking dope in here." I would not have admitted it, thats too easy. I simply remained calm and told them I had no idea what they were talking about, after all there was no actual smoke. Make up some bullshit excuse even if it is completely improbable or obviously a lie. Im pretty sure I told them it was just cologne or air freshener. After the interrogation and some snooping which turned up nothing I was told that if they smelled smoke in my room again I was to be grounded and a week later was threatened with a drug test, which turned out to be an empty threat.

At school when a teacher saw me about to spark a bowl in an alcove directly outside a set of doors I did the same thing, denied everything, and said I didn't smoke and that I went out that way to get to my locker...which was a very stupid excuse since the door I would have entered into only leads up to the same doors we had come out of. Nevertheless he told me he was going to send a notice to my administrator, which he never did.

Never say more than you have to, and in many cases you'll find that you didn't need to say anything at all. (Less is more)