At Northern High School in Dillsburg, Pa., Principal Steve Lehman’s locked safe, which once contained the occasional pack of confiscated cigarettes, is now filled with around 40 devices that look like flash drives.

The device is called a Juul and it is a type of e-cigarette that delivers a powerful dose of nicotine, derived from tobacco, in a patented salt solution that smokers say closely mimics the feeling of inhaling cigarettes. It has become a coveted teen status symbol and a growing problem in high schools and middle...