Between the 2002-3 and 2010-11 academic years, the number of high school students taking college courses for credit increased by 68 percent, to nearly 1.4 million. By 2015, nearly 70 percent of high schools offered dual enrollment, according to the Government Accountability Office. The explosion has been a boon for the nation's community colleges, which have seen adult enrollments plummet since the end of the last recession. At some two-year colleges, high school students now make up half of enrollments, according to The American Association of Community Colleges. The rise of dual credit has benefitted students, too. Research shows that students who take dual credit courses are more likely to enroll in and complete college than students who don't--and to finish faster, too. A few studies have found disproportionate benefits for low-income students. However, dual enrollment is creating some financial challenges for four-year colleges, which depend on large lecture-based introductory courses to subsidize more expensive upper-level offerings and to recruit students to majors. With more students knocking off core courses in high school, core 101 classes are shrinking, and humanities departments are losing money and majors. Meanwhile, skeptics ask whether the courses--which are often taught in high schools, by high school instructors--are truly comparable to college classes. In addition, racial and socioeconomic gaps in access to and enrollment in dual-credit courses have led some school and campus leaders to question whether the programs may be inadvertently deepening a divide they were supposed to bridge, giving a head start to students who would have attended college without them.