July 31 (HealthDay News) -- In the past 17 years, there has been an overall decrease in risky sexual behaviors among U.S. teens, a new government study shows.

The rates of having sexual intercourse or multiple sex partners have been dropping, while the use of condoms has been increasing, U.S. health officials reported Thursday.

However, the rates of risky sexual behaviors among some groups, including blacks and Hispanic students, hasn't changed since the 1990s, according to this week's issue of Morbidity and Mortality Weekly Report, a publication of the U.S. Centers for Disease Control and Prevention.

"Progress has been made in the last 17 years in decreasing kids' risk for HIV- and STD-related infections," said study co-author Laura Kann, chief of CDC's Surveillance and Evaluation Research Branch in the Division of Adolescent School Health. "However, despite the positive changes, many students still engage in HIV- and STD-related risk behaviors."

Using data from the CDC's Youth Risk Behavior Surveys, Kann's team looked at sexual behavior among high school students from 1991 to 2007.

During that period, HIV-related sexual risk behaviors decreased. The prevalence of sexual experience dropped by 12 percent, from 54.1 percent to 47.8 percent; the prevalence of having multiple sexual partners decreased by 20 percent, from 18.7 percent to 14.9 percent; and the prevalence of current sexual activity went down 7 percent, from 37.5 percent to 35 percent.

In addition, the use of condoms among sexually active students increased 33 percent, from 46.2 percent to 61.5 percent, Kann said: "All that is good news."

But some groups haven't seen a decrease in risky sexual behavior, Kann added. "For example, black students still have the highest prevalence of sexual experience with multiple sex partners and current sexual activity than any of the other subgroups," she noted.

The prevalence of sexual experience among black students hasn't decreased since 2001, Kann said. "Among Hispanic students, we didn't see any changes since 1991 in sexual experience, multiple partners or current sexual activity," she added.

Among male students, the prevalence of sexual experience and multiple sex partners hasn't gone down since 1997, Kann said. "Current sexual activity didn't change since 1991," she said.

More effort is needed to reach black and Hispanic male students, Kann said. "We really need to focus our efforts on these groups in particular, but not to the exclusion of everyone else," she noted.

Another report in the same publication called for state and local education agencies to encourage schools to provide HIV prevention education. In addition, they should work to increase the number of teachers who teach HIV prevention, the report added.

In 2006, most high schools who participated in the CDC survey said they offered some HIV prevention education. However, few schools taught all the 11 topics covered by the CDC's program, according to the report.

Moreover, schools should have a policy to help protect the rights of students and faculty who are HIV-positive.

SOURCES: Laura Kann, Ph.D., chief, Surveillance and Evaluation Research Branch, Division of Adolescent School Health, U.S. Centers for Disease Control and Prevention, Atlanta; Aug. 1, 2008, Morbidity and Mortality Weekly Report