The difference between an all-white school and an all-minority school is usually that the all-white school is discriminating for the wrong reasons. The all-minority school is usually in place to try to maintain the culture and language that might easily otherwise be gobbled up by the mainstream society (whatever its race). So this is an instance of apples and oranges.
The Boy Scouts wants to be an all-straight organization and it can be because it is private. But it can't participate in a lot of public school venues and other public arenas now because of its over policy of discrimination. I think the difference is that to set up an organization to exclude others who are considered "inferior" is different than trying to avoid cultural genocide. Why does the white student want to get into Kamehameha? Probably so his daddy can bring a big lucrative lawsuit.