Most people who take the SSAT from public school usually get disappointing scores. I, for one, did not get a good score the first time I took it (around the 50-60% percentile). I always scored above 90 percentile in my state standardized testing (NJ). I come from a low income town and my scores were being competed against kids who had private tuition their entire life and their parents bankrolling their elite education with tutors, prep centers, etc. Trust me, boarding schools know and holistic admissions alleviates this fairly well.
But, I do recommend studying for the SSAT. It usually is just a different learning style that you can easily master. I was able to bring my scores to the ~80s and 90s just by grinding an SSAT book I got for 10 bucks and quizlets for vocab.