In theory, random drug testing should deter drug use in test subjects for fear of getting caught. Apparently it doesn’t, at least not in schools where testing teenagers for marijuana use is commonplace, according to a new study from the University of Pennsylvania’s Annenberg School of Public Policy.

An estimated 20% of U.S. public schools employ drug testing as a means of limiting alcohol, tobacco and marijuana use in students. The researchers interviewed 361 students (about a third of whom went to a school with a drug testing policy) and compared usage data from schools with the policy against those who have what they refer to as a “positive climate.” The result? For most students, the only thing scary about a drug test is having to use the bathroom in front of a supervisor.

“The bad news is that a policy of drug testing has no effect on students starting to use alcohol, cigarettes or marijuana,” said Dan Romer, associate director of the Annenberg Public Policy Center (APPC) at the University of Pennsylvania. “There’s also no effect on escalating the use of those substances.”

On the other hand, students in schools with a positive school climate reported a lower rate of starting to use cigarettes and marijuana, and a slower escalation of smoking at the one-year follow-up interview. Students in schools with positive climates were 15% less likely to start smoking cigarettes and 20% less likely to start using marijuana than students at schools without positive climates.

So, what exactly determines a “positive school climate?” Essentially, it seeks to create “good relations between teachers, students, and parents – the triad of communication,” says Romer. In addition to things like life-skill training courses aimed at limiting destructive behavior, schools reinforce positive climates “by engaging students in the decision making that goes on in the school, making sure that they feel a part of it, and that if discipline is needed that it’s done in a way that’s not arbitrary but is explained,” Dr. Romer adds.

In schools that employ both drug testing and a positive climate, the testing appears to be more of a deterrent, though only in female students. The study also suggest that the way schools go about administering tests has an impact on their efficacy. When used as part of a larger health initiative, rather than a means of punishment, may be more effective, according to Desirae Vasquez, director of program services for Freedom from Chemical Dependency in Newton, Massachusetts.

Regardless of testing policy or climate, one thing that’s apparent is that teenagers still like to party: Neither drug testing nor school climate affected the start of drinking alcohol.