My father taught at a local state university, nowhere near state flagship level but still a "university" in name, and he occasionally had kids in class who were close to functionally illiterate. His reading for the semester was very light (something you would get through in a competitive university in a week) but from helping kids who came to his office he could tell it would have been a hundred hours of reading for some of them, even if they skipped all the words they didn't know.

I think I approve of letting every kid take a crack at college. If they graduate high school, there should be a school somewhere that gives them a chance. The problem is that if they aren't ready, there's no place they can go. They can't go back to their old high school and demand to be taught what the teachers pretended to teach them. The best they can do is take remedial reading classes at the university, borrowing money to learn what their local schools were supposed to teach them for free.

Most (maybe all?) of the kids like this that my dad encountered came from rural schools where you have a similar range of preparedness and home situations that you see in an urban setting, but you only have enough kids the same age to fill one or two classrooms. A teacher can't personalize the curriculum for every single student, so a kid who falls significantly behind will, after a certain point, no longer receive a meaningful amount of instruction, because the curriculum that's appropriate for the bulk of the class is beyond what they can engage with. Schools that recognize the unfairness of failing a kid that they're not even teaching tend to pass these kids along from grade to grade and then graduate them, and it's hard to fault them for it. If you think of instructional level as a spectrum from remedial to advanced, rural schools only have the resources to cover the middle part of that spectrum where 90-95% of their students are. All they can do for the rest is give them an apology and a diploma.