Ironically enough the more I think about it, I would think it's easier for a kid from a rough/disadvantaged household but good potential to get a good ACT/SAT score rather than GPA. GPA requires 4 straight years of full attention, attendance, and discipline. That's really hard for someone with a rough home life or issues, could be a year or 2 in there that really fucks your cumulative score up. You need support from teachers and your parents for years.
Testing on the other hand, if you're naturally smart and bright, is a matter of applying yourself for a couple weeks of studying and just taking it. ACT/SAT study guides are freely and cheaply available IRL or on the internet at this point, you're either going to be able to get it done in the allotted amount of time or not.