Marijuana use among California students dropped over the past couple years, even as laws legalizing the drug for adults 21 and older started to kick in, according to the latest state-commissioned California Healthy Kids Survey.

One possible reason is that students told surveyors marijuana is harder to get now than it was a few years ago.

Legalization advocates are calling the results early evidence that regulating marijuana protects kids better than banning it — a pattern that has so far played out in other states.

“These initial reports confirm that legalizing and regulating cannabis doesn’t increase youth marijuana use, but rather it has the opposite effect,” said Ellen Komp, deputy director of the California chapter of the advocacy group NORML.

Despite such claims and the initial poll results of reported cannabis use, some researchers remain worried about how students perceive the potential harm of marijuana, with students ranking it far less dangerous than binge drinking and tobacco.

Both researchers and legalization opponents also say better data might come in the next survey, which will cover the first year and a half of legal marijuana sales in California.

“The reality is the majority of the data presented in this study were collected at a time when recreational marijuana use was not legal,” said Kevin Sabet, president of Smart Approaches to Marijuana, which fights legalization efforts around the country. Sabet noted that retail sales became legal in select California communities only during 2018.

The California Healthy Kids Survey is commissioned by state health officials every two years to gauge student needs. It gets data through self-reported information from 45,264 students in seventh, ninth and eleventh grade. The survey released this week covers the 2015-2016 and 2016-2017 school years.

Californians in November 2016 passed Proposition 64, which let adults possess up to an ounce of marijuana and to grow up to six plants at home for personal use. Though legal recreational marijuana sales didn’t start until Jan. 1, 2018, personal rights granted by Prop. 64 took effect immediately, creating a nearly eight-month overlap with the period covered in the latest student survey.

Marijuana remains the second most likely mild-altering substance students will try, behind alcohol. But the survey says junior high and high school use of both substances continues to fall at what researchers termed “striking” rates.

Seventh graders who reported using marijuana within the past 30 days dropped from 5 percent in the 2013-15 survey to 2.3 percent in the 2015-2017 survey. The number of ninth graders who reported so-called “current” use dropped from 13.4 percent to 9.5 percent, while the current use rates for eleventh graders fell from 20.1 percent to 16.7 percent.

The number of students who reported using marijuana at least once also dropped from the previous survey by 3 to 6 percentage points.

The survey also found that students who do consume marijuana are using it less frequently than in previous years. The number of eleventh graders, for example, who said they used marijuana 20 or more days in the past month fell from 5.3 to 3.9 percent.

The California findings are consistent with studies from other states that have legalized recreational cannabis.

In Colorado, the numbers of teens using cannabis at least once and regularly have essentially held flat since legal marijuana sales started in that state in 2014, according to a study published in May in the International Journal of Mental Health and Addiction and a second study published last month in the journal Prevention Science. And other than a slight increase in Alaska, the most recent National Survey on Drug Use and Health showed declines in teen marijuana use for the three other states (plus Washington, D.C.) where cannabis was legalized by June 2016.

But Sabet said there isn’t reliable baseline data on the rate of youth marijuana use in those states prior to legalization, insisting survey methods have evolved. So he’ll be watching the “more robust data sets” expected to come out over the next few years.

Since teen marijuana use in California remains higher than the national average, Scott Chipman with Citizens Against Legalizing Marijuana says he hopes to see more state-sponsored messaging aimed at discouraging teen use.

State Superintendent of Public Instruction Tom Torlakson agreed there’s more work to be done, saying the state must “be diligent” in its efforts to “prevent, or at least limit, marijuana use” for teens in the wake of Prop. 64.