I would challenge if ACT/SAT is much about pure intellectual merit, considering how much one can boost their score with dedicated tutors and so on, but I digress. How to show intellectual merit other than standardized tests without relying on rich parents:
(a) Straight As
(b) (if applicable) Take all AP courses available at your high school
(c) (if applicable) Take additional college courses through community college dual enrollment or online school
(d) (if applicable) Join the math club, honors society, run for some kind of student council position, or other academics-oriented or leadership-oriented club role that doesn't require a hefty buy-in
b-d may or may not apply, depending on your school, school district, and state.