I got a fairly good score on my first SAT practice test going in blind and didn’t need to prep much for my actual exam. At least at Lawrenceville, the curriculum covers everything that you need to know for the math sections, and they teach us grammar in English class with (among other reasons) the explicit intention being “you will need to know this for your standardized tests.” Reading comprehension is mainly based on practice, which you definitely get a lot of through Harkness English class.