What is the purpose of school instruction?

Ok, so I know that this question is pretty big, but I've been struggling with it for a long time. In Albertinti's chapter 24, Teaching of Writing and Diversity, he says, "The research reviewed in this chapter has demonstrated that achievement, access to instruction, and identity development as a writer often relate to ethnicity, race, gender, and disability." and "As we have seen, learning to write convincingly in accpetable discourses, is extremely difficult for students marginalized by ethnicity, race, class, gender identity, or disability."

If we believe that these "diverse" groups have discourses that are meaningful and powerful within their own community, I struggle with why we have to change all that just to acclimate them to the majority, middle class, American (US) culture. Of course I want diverse students to have success, but at what point do we perpetuate the problem by reinforcing its power in school?