“ Even the Court of Appeals' requirement for standing would be met on remand since petitioners' transfer to the white high school would desegregate their grades to that limited extent. Moreover, we reject the Court of Appeals' view of standing as being unduly restrictive. Two theories would give students not yet in desegregated grades sufficient interest to challenge racial allocation of faculty: (1) that racial allocation of faculty denies them equality of educational opportunity without regard to segregation of pupils ”