The formative assessment approach organizes classroom work around teachers and students (and often parents) understanding how current work compares to important learning standards and planning next learning steps based on that information. The research behind that approach shows that, in addition to providing the largest gains for the students who struggle most.
In what I've call the "sunlit vision" of that kind of classroom, that strong, shared approach allows a virtuous cycle of rising results, in which students, teachers, and parents, see results that build confidence, develop confidence that promotes further results, and are able to generate impressively higher levels of achievement.
In situations where the ugliest gaps persist and deepen, I suspect an alternative, far less healthy cycle is at work. In that version, teachers doubt that students can succeed, students and parents doubt that teachers intend to help students succeed, everyone can smell everyone else's despair, and every round of student work becomes further evidence that there's little point in hoping and aiming any higher.
I've heard too many educators and citizens say too easily that "some kids" or "our kids" or "you know, those kids" won't be able to meet any higher standards than their current grim level of achievement. Often, the kids in question are from minority backgrounds, but the same phrases are applied to children from low-income homes, children with disabilities, and even children who live in "urban" settings. Often, the people saying those things would be terribly upset to hear their words described as prejudiced, hurtful, and ignorant--but they really believe there is no basis to believe anything better is possible, and they really are mistaken in that belief.
Formative assessment, understood as a rich classroom process, is the practice I think has the best chance of breaking that cycle, stopping that talk, and sustaining the work needed to deliver on each and every child's birthright to learn and grow into adult success.