I think this depends on the state and the university. In the state I live in (MN) there is a program called Post Secondary Enrollment Options. I think some other states use the same term, too. Students can take some or all courses in the state university system during their junior and senior year of high school. Tuition fees are paid for by the state. I believe it is pretty competitive to enroll in courses at the state flagship, and you have to be aware of deadlines and paperwork required (you can't decide summer before junior year that you want your kid to take classes at the flagship that fall and get it done in that time period, I think the deadlines are earlier).

Here is a link to Minnesota's program:

http://www.mnscu.edu/admissions/pseo.html

One thing to be aware of is that if your student has completed essentially two years of college work prior to enrolling in college, they may have to apply as a transfer student instead of a freshman. Different admission criteria, fewer slots, and usually no merit aid available. Which may be worthwhile for the overall cost reduction, but sometimes parents are surprised by this limitation.

Also, in most states students still have to fulfill the high school graduation requirements, and depending on the state that can limit your ability to do dual enrollment in college courses (if they don't count toward the graduation requirements -- it depends on how the state has defined them).

Some colleges (especially top universities) do not accept some or all of these credits -- you need to read the information on a given college website carefully about acceptance of other credits. You can be pretty sure that if your student stays within your state university system to complete their undergraduate degree that the credits will be accepted. Outside that, your milage may vary. Be sure your student saves the course description and syllabus for any courses taken in case the colleges considering accepting the credits want to see it.

We didn't actually do this. My kids' private school did not allow participation in the PSEO program, although at one point we looked into withdrawing D2 from that high school, switching to the public school, and doing PSEO for her last year of high school. But didn't end up going down that path. Others out here may have more experience.