Free college sounds great, until you realize that college will be rationed, probably by academic achievement, which would block students from under-performing K-12 schools from entering college. Is that the plan? Or are we going to fill colleges based on racial, gender, or economic factors?
It would not entirely block students from underperforming K-12 schools, because college admissions officials do not solely look at the performance of schools. They also look at things like raw GPA, class rank, classes that are acknowledged as being more challenging, and on other aspects of scholarship like applications essays and standardized tests.
Certainly there would be fewer students from underperforming schools, because there would generally be fewer strong candidates from underperforming schools, through the sheer nature of the statistics of what led to those schools being labeled as underperforming to begin with.
Thing is though, that's why the higher ed system is tiered. No one looks on Truckee Meadows Community College as being at the same level as UCLA, and no one looks at UCLA as being on par with Yale. There are or at least should be options for many talented high school graduates who are looking for college. Not everyone gets to go to MIT. Not everyone even gets to go to the University of Arizona. Some end up at Chattahoochee Technical College. And that's okay. They might well be able to transfer if they excel in their new environment after high school. If not, they might well have to go through the programs that they can perform at.
What we do need is better assessment for certification of programs, and weeding-out of faux-colleges that themselves just exist to profit off of student loans for 'students' that will never graduate, particularly private for-profit colleges. They need to be held to minimum graduation rates based on original enrollment, and if that means compelling them to find fraudulent enrollments that likewise are attempting to game the educational financing system, then they need to step up.