There are boarding schools that prioritize the sports part of the equation and allow their kids to train at very high level all year. By and large they do this but taking academics down a notch (or two). At the academic boarding schools the strongest athletes are typically day students who either have family schlep them around or last couple years have a car to drive themselves places after class. By junior year it gets easier to waive the school sport requirement if you are seriously involved in something else. But as @one1ofeach said it is still a grueling grind since the academic workload is massive and if sports are the priority there are much better options.
This is the case for kids entering in 9th grade, anyway, the equation is different for PGs or even kids who enter as repeating 11th graders.