Who is Karla from Stone Butch Blues and what is their importance?

Karla is Jess's friend at high school. When Jess sits down at Karla's table at lunchtime, everyone in the cafeteria freezes. Why? Karla is black, and the cafeteria is strictly segregated - that's the way most students want it, and that's the way the principal, Mr. Donatto, wants it, too. Jess is suspended for one week simply for sitting on the black side. Of course, Karla gets suspended for two weeks. This injustice, along with the rape, makes Jess decide that she can no longer stay in this school another minute.