Some 46.8 percent of students said they engaged in sexual intercourse in a 2005 survey, down from 54.1 percent in 1991, according to the report.

Some 14.3 percent of students in 2005 said they have had multiple partners, defined as sex with four different people during one's lifetime. That figure is down from 18.7 percent in 1991.

At the same time, the number of students who say they used a condom the last time they had intercourse rose to 62.8 percent in 2005 from 46.2 percent in 1991, the survey said.

The report was based on student responses to anonymous, self-administered questionnaires in public and private schools in all 50 states and the District of Columbia by the CDC's Youth Risk Behavior Surveys.

One aim was to determine the extent to which U.S. students were at risk from HIV or other behavior-related illnesses. High school students are generally aged 15 to 18.

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