I imagine through either scholarships or taking on bunches of debt. I was nowhere near as aggressive as some other classmates, but I managed to grab ~20k of total scholarships that helped me out immensely for college. Still a small chunk of what was needed for college, but it was an option I had compared to high school just making me eat the admissions and AP test costs wholesale.

Fortunately my parent did cover these costs, but I know others weren't so fortunate.