DANVERS — The brown SUV rolled to a stop in a Holten Street parking lot as two 17-year-old high school students in the back seat emptied their pockets of everything except their cell phones and a $20 bill. They wouldn't need much more to buy a pack of cigarettes in Danvers. The girl, wearing a gray hooded sweatshirt and a pair of jeans, stepped into the pouring rain, approached a sales clerk at the Mobil gas station and returned moments later with a pack of Marlboro Reds. The two teens work undercover for the North Shore Tobacco Control Program, the...