(1) An internet ready device.
(2) Reliable internet service.
(3) A quiet environment where they're able to study undisturbed.
(4) Have the time available to study, no younger siblings to watch over, and don't have a strenuous paid job.
For many lower income kids studying for the SAT isn't practical, even if the material is available at no cost. Khan Academy certainly helps a lot of students that can't afford expensive tutors or private classes, but it's by no means a complete solution. We need to do more as a society to help disadvantaged students prepare for college.