For the report, the University of Michigan researchers surveyed more than 42,000 kids in grades 8, 10 and 12.
Researchers also found that fewer high school seniors were smoking regular cigarettes, a drop from 8% to 6%.
No data were available on how many students are vaping marijuana, the AP said.
The study was published online Sept. 18 in the New England Journal of Medicine.
The same trends were reported in a government survey last week.
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