An immersive year in high school can put one much further down the path of cultural and linguisic fluency. And some kids may want to do both.
As someone who grew up outside the US, I definitely see the value of doing this sooner.
Most college programs really create enclaves of American students studying abroad – like extended tourists.
Two different beasts. Imagine the difference in experience between someone coming here to study at a school for a year vs going on a tour around the US with a guide. Both great experiences but not the same.