The three-year survey found that teens who had oral sex by the end of ninth grade were at the highest risk of having sexual intercourse during high school. These teens had a 25-percent chance of having intercourse by the end of ninth grade and a 50-percent chance by the end of 11th grade.

Meanwhile, teenagers who did not have oral sex until the end of 11th grade had only a 16-percent chance of having intercourse by the end of that school year.