San Francisco has more (drug addicts) than (students enrolled in highschool)
not
San Francisco has more (drug addicts than students) enrolled in highschool.
I'm ashamed to admit I had to follow all the way to the source PDF to figure that one out.
Unless them being enrolled, but not attending class because they are drug addicts makes them not students.
That didn't clear it up for you?
So, it probably would have cleared it up, had I decided that "Marginal Revolution" sounded like a reliable source, or if it had been a bit longer. I skimmed, and that's on me :/
San Francisco has more drug addicts than San Francisco has students enrolled in highschool.