Who should public schools serve?

Mary Battaglia’s post on a Curious Social Development began to raise some fundamental questions about who the public schools should serve, and a healthy discussion on this blog might help clarify differing views on some of the current trends being implemented in the MMSD.
To set the discussion, let me offer that schools serve roughly three populations: the neediest, the average, and the brightest — not too startling a breakdown.
Which group should the schools target?
If schools target one, should the schools just let the other two groups fend for themselves? For example, should schools place their priorities on raising the academic accomplisments of the lowest performing students, on the assumption that the average will get by and the brightest will succeed regardless of what schools do? Or maybe schools should triage the groups? Let the neediest drop by the wayside, target the average, and just expect the brightest to do well.
How then do we determine whether the schools succeed? Do we measure success by the number of National Merit Scholars or by the increase in the performance of the neediest?
I think schools should and can successfully serve all three groups by using curriculum and programs to challenge all students academically.
I hope that many will candidly post their opinions.
ps. To the grammarians, I know that the title should be “Whom should public schools serve?” But “whom” sounds so stilted.