They can be as furious as they want, but remeber that this mistake doesn't mean they don't still care and love you. That's how I see it anyways. Mistakes are part of human nature. You'll get through it, but try to have a mature discussion about what happened. If they start yelling and screaming, cursing, then tell them, "I'll have this talk when you feel like talking, not yelling. I made a mistake, I feel awful about it, and I won't bring cannabis to school anymore," or something like that. Admit the mistake and you might be surprised of how they react.