Marijuana use among high school seniors has risen to a 30-year peak, with one of out every 15 high school students now using the drug on a daily basis, according to a new government survey released Wednesday by the National Institute on Drug Abuse.

Overall, the survey conducted by the University of Michigan found that some 25 percent of the 47,000 students from the 8th, 10th and 12th grades who participated said they had used marijuana in the past year – an increase of about 4 percent over the past four years.

Many of them, including one in nine of high school seniors, also reported for the first time in the institute’s annual “Monitoring the Future” survey that they had also tried “synthetic marijuana,” a mixture of chemicals and herbs known as “K2” or “spice.” Up until recently the mixture could be sold legally in stores as herbal incense.

Lloyd Johnston, the lead investigator in the study, attributed the increase in “pot smoking” in part to more relaxed view among teenagers about the risks of using marijuana, which has been made legal in some states for medical purposes such as treating glaucoma and managing pain.

But he indicated that the results of the survey were still somewhat surprising, given the significant decline this year in cigarette smoking and alcohol use among teenagers. His report on the survey also noted a decline in the use of other drugs as well, including cocaine, crack, prescription drugs and even over the counter medicines like cough syrup and other cold remedies.

Still, he said there’s no getting away from the fact that “one in every fifteen high school seniors today is smoking pot on a daily or near daily basis.”

“That’s the highest rate that we have seen over the past thirty years – since 1981,” he added.