That is the real question.
There is an argument to be made that certain boarding schools actually diminish your college prospects.
If you’re thinking Andover/Exeter types, you’re really good at what you’re good at in your current school. You go to those places and chances are you won’t be the best student, athlete, musician or artist, etc. So you go from being exceptional at your current place to being in the middle of the pack (or worse if you jump in 11th grade). AND you’ll be working harder.
@gardenstategal asked why you’re thinking about it now. I’m also curious.