High school girls are more likely to exercise vigorously if they're in girls-only physical-education classes that offer a variety of activities such as dancing, aerobics and brisk walking, a new study reports.

And they do better if the school in general promotes activities for girls, says the report in September's American Journal of Public Health.

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Pate and his colleagues chose 24 high schools associated with 31 middle schools in South Carolina. The researchers measured the height, weight and activity level of 2,744 girls in eighth grade and again in ninth grade.

Ninth-grade girls at 12 of the high schools took traditional co-ed physical ed classes. At the other 12 schools, the girls participated in such activities as aerobics, dance, exercise walking, self-defense and strength training in addition to organized sports. In most cases, these classes were girls-only, which many girls like because they are self-conscious around boys, Pate says.

Also, at the non-traditional schools, photos of girls doing activities were posted on bulletin boards.