My son’s writing and reading skills really blossomed while in boarding school. He wouldn’t have gotten nearly the same attention at our local public school. The academic rigor at boarding school really set him up for success in college. The first year of college was very easy for him.
But as pointed out above, attending a rigorous private/boarding school can make it difficult to get top grades. The Academic Index is very formulaic, and no adjustment for level of difficulty in grading. Fortunately the AI weighs heavily on standardized test scores.
For sports like Golf, Squash, Fencing, Sailing, etc. the coaches rely on high academic indexes. This offsets sports like football, hockey, basketball where the AIs are lower. So having a low AI in a sport like golf could cause problems.