Being young at BS is rough. It is an environment that requires a lot of maturity to navigate successfully so kids who were previously accelerated, or on the tail end of their class, benefit greatly from repeating. I cannot stress this enough. There is little supervision and there’s lots of “mature content!” If a kid doesn’t know how to deal its going to be hard for them. There’s also much less social scaffolding than parents might expect, more kids thrown together and expected to be mature. There are always issues, often stemming from the insecurities (and therefore nastiness) of the less mature kids.
Additionally, entering top schools after 9th grade is academically more challenging. 9th grade is by no means easier but it’s generally acknowledged that having your grades dip freshman year while you learn to navigate BS is ok. A similar dip sophomore or junior year is a bigger deal.
The kids we know who have entered Groton after freshman year have struggled to get their legs under them academically.