Are you more upset that he smoked marijuana or that he hid it? How would you have reacted if he had been open about smoking? It's important to understand what you're punishing him for because otherwise you risk alienating him.

I don't mean to trivialize any of this, just to say that at 15 years old, typical American males are experimenting with things like marijuana or beer.

Instead of instantly punishing him, you should take the opportunity to get him to open up about why he's doing what he's doing. The punishment can come later, not as an iron fist (presumably for his own benefit), but as a realistic consequence for his actions. At the end of the day, he lives in your home and if you don't allow marijuana use, he should be punished for not respecting those rules. But if you get into the punishment first, he'll clam up so tight you'll lose a line of communication to your son.

This is spoken from personal experience. My mother was more upset about marijuana than cigarettes, but then again everyone was still suffering reefer madness back then.

Good luck to you both