The biggest being that a comparable course cannot be offered at the High School. This is intepreted quite loosely so you won't get a college level speech course paid for if a High School equivalent exists (even if it does not grant college credit).
High Schoolers have several ways to access college courses (including university courses):
1. Dual enroll in which the course is taught by the High School teachers but college credit is granted at the local community college.
2. AP Courses (not a college course but can get college credit by a year end test on the subject).
3. PSEO (Post Secondary Education Options) in which the student brings a description of the desired course to the counselor, and the counselor approves/disapproves taking the course.
Financially PSEO is hard on community colleges. My daughter's Calculus textbook would cost $40 to rent (would have been more last semester) and it is provided free of charge. Her Physics book would cost $120 to rent for a semester, and it is also provided free of charge. Each of these 4 credit hour courses only have a $250 tuition that the local school district pays to the community college. So the community college gets $500 for providing approximately $1400 of services.
I don't buy the 40-50% participation. Participation rates for PSEO/Dual Enroll/AP are not that high at our High School. Many kids are not able to take college classes until they complete their senior year. It makes little sense to take what would be viewed as remedial classes (for example a class before Precalculus) at the college when the High School should be offering this class.