"Early initiation of sexual activity is associated with having more sexual partners, not using condoms, sexually transmitted infection and pregnancy during adolescence," the report noted. It called the falling rate of sexual activity among 9th- and 10th-graders "especially encouraging."
The researchers said they could not attribute the trend "directly to any specific intervention," but experts have previously cited a number of factors, most importantly access in school and online to straightforward information about sex and contraception.
Laura Lindberg, principal research scientist at the Guttmacher Institute, a nonprofit organization that studies reproductive rights and health, noted that the vast majority of the decline occurred between 2013 and 2015.
"So we need to see if this is a short-term blip or this is something that is going to continue," Lindberg said. "The drops are very large in 2015, and that raises questions of survey value."
The results differ from another national survey, conducted in teenagers' homes, that showed little change in sexual activity. The National Youth Risk Behavior Surveys cited in Thursday's report are conducted in schools.
Still, she said, the finding that ninth- and 10th-graders are delaying sexual initiation is a welcome development that most likely results from the end of federally funded school programs that taught abstinence until marriage. In 2010, she said, the Obama administration replaced that curriculum with "medically accurate" information about sex and contraception.
In July, the Trump administration cut funding by more than $200 million for groups across the United States that work to prevent teenage pregnancy.
"The big takeaway for me here is that even with the observed delay in sex, by the time they graduate high school, it's still the case that more than half of students have had sex," Lindberg said. "So we need to do what we can to encourage delay and support healthy choices" when teens begin having sex, she said.
One of the study's authors, Kathleen A. Ethier, director of the CDC's division of adolescent and school health, said in a statement that "we do know that sexual risk, substance use and other behaviors have common risk and protective factors, and that youth development approaches, parental monitoring and connectedness are protective across these behaviors and experiences."
She said more research is needed to understand which factors have contributed to the decline in teen sex and pregnancy.
Overall in 2015, 41.2 percent of high school students reported having had sexual intercourse, down from 46.8 percent two years earlier, the same rate found in 2005, according to the CDC's National Youth Risk Behavior Survey.
More strikingly, 48.5 percent of black students in the survey in 2015 said they'd had sex, a steep drop from 60.6 percent two years earlier and 67.6 percent in 2005. Over the same two years, sexual activity among Hispanic high-schoolers fell from 49.2 percent to 42.5 percent. Among whites, there was a smaller decrease, from 43.7 percent to 39.9 percent.
Among ninth-grade boys, 27.3 percent said they'd had sex before, down from 32 percent in 2013 and 39.3 percent in 2005. For ninth-grade girls, 20.7 percent in the 2015 survey had begun having sex, a sharp decrease from 28.1 percent in 2013 and 29.3 percent in 2005.
The CDC report looked at data from 29 states that had conducted the survey and found some disparities among them. But only two — Wyoming and North Dakota — found no decline in high school sexual activity over the decade.
Other reports show that teenage pregnancy and smoking hit all-time lows in 2016, with just 11 percent of teens saying they had smoked a cigarette in the previous month. Alcohol and marijuana use has also declined in recent years, though not as sharply.