It is typically easiest to work through the school, as schools don't need to provide nearly as much documentation as individuals do. Best is if you have at least three years of documented 504/IEP accommodations of the type you are requesting (mainly to prove that this isn't a recently "discovered" disability to bump up test scores after an initial unsatisfactory result).
Grades aren't a major factor in accommodation approval anymore. They weight the 504 and IEP a bit more than in the past, although they seem to be leaning pretty heavily on testing to document 100% extended time (not 50%--that's relatively easy, if it's on the 504/IEP).
Definitely good to start early and follow-up frequently.
