He's 12 - you don't spank a 12 yr old.
They don't listen much at this point. The school did not find it so they can't do anything and it would have NO EFFECT on him at all if they did find it and deal out the punnishment. It only hurts the parents.
That said, here we go - I would advise you need to take a double dose of self control and talk with him calmly at the table. Explain the issue and stress the consequences if this happens again. Tell him about how you will feel if he does this again and how it will put a burden on all of you because of his disrespect for the rules. You need to get your point across to him that this is not acceptable and that he is responsible for his actions. Either he cares about you, or he does not. If he gets the message then fine. If he does not get the message, then you will have to let this all play out.
He either gets it now or he gets it later.
Be strong!
-Mark.