It is easy. You tell them in plain English. I think you are a good kid and even good kids make mistakes. The school told me you tested positive on the spray test. I told them you were a good kid and that we will go to the Dr office to clear your name once and for all. If the kid is innocent, he will be happy to oblige. If he is a pothead, he will resist.
Your source of resistance is your personal guilt of being a pothead and not wanting to be caught yourself. You cannot imagine what an innocent person would think like because you are not innocent yourself. I happen to speak from a different position than you do since I have never used.