Some 1.3 million students took classes for university credit before completing high school during the 2010-2011 academic year, up 67% since 2003, according to the U.S. Department of Education.
Much of this trend is being driven by the skyrocketing cost of college. Students enrolled in early college high schools, for example, earn an average of 36 college credits, nearly a third the number they'll need for a bachelor's degree, according to a study by the advocacy group Jobs for the Future.
But there's also evidence that exposing high school students to the challenges of college-level work can increase their eventual likelihood of success.
More students who take college-level courses in high school go on to college than their classmates who don't, a report released in June by the American Institutes for Research, or AIR, found.