How many high school students are having sex? A 1999 study by the Centers for Disease Control and Prevention found that 65 percent of 12th-graders and 39 percent of high school freshmen were sexually active. Overall, 52 percent of high school-age boys and 48 percent of high school girls were having sexual intercourse. About 80 percent of 19 year olds have had sex. These facts are taken from a nationwide study taken in 1999 and the trend is going up. Teen pregnancies occur very often in the United States. Eighty-four out of every 1,000 teen girls between ages 15-19 become impregnated. T…