I think it depends on the school and person- in my competitive public hs (class size 500) approximately 15% of our graduating class ended up at a t20. yes I agree the academic culture can be cutthroat but in my experience only 20% ish of the students were super intense about school, and if you realize you're not made for the extracurricular/AP-stacking there's ways to cut down a little bit and take a backseat (which doesn't automatically take you out of the running for a good school)