The Fairfax Co School System has a good reputation.
Faairfax is like a lot of well-to-do suburban district: the good students are—practically speaking— tracked, so they never see the losers except accidentally. Ship passing in the night.
That's an understatement. They are arguably the best public school district in the country. The high school in Fairfax County (Langley I believe) competes academically with 20k-30k a year private schools in the area. What's even more interesting, the Fairfax county school system spends significantly less per pupil than does DC Public, with unbelievably better outcomes.
Duncan’s daughter, however, is enrolled in Arlington County public schools. Much smaller school district, but has some excellent schools.