Lastminutemom has it right. DeKalb has some wonderful schools as well as some abysmal ones. Schools around Dunwoody (Austin, Vanerlyn, Chesnut) are generally pretty good as are schools around the Oak Grove and Fernbank areas.
We are in Decatur. Decatur uses Expeditionary Learning in its K-3 schools (google ELOB). Their 4th/5th grade school will "officially" be an International Baccalaureate school by this spring I believe - although they have been running as an IB school for a couple of years. The middle and high school will be IB schools in a few years.
Decatur gives all the required state tests, but they don't focus on the test very much in comparison to other schools b/c IB and ELOB don't lend themselves to that type of work. They do get a little nutty about it after spring break though - for the two weeks before the CRCT... but they are back to normal by May.
City of Decatur schools are Clairemont, Oakhurst, Winnona, Glennwood, Renfroe Middle and Decatur High. You can check out their scores, etc. here....
The Governor's Office of Student Achievement
You can also get some good info in greatschools.net ... on both private and public schools.
You can also check around Dekalb on the same website.
Private vs. public is a tough decision. Honestly, if we won the lottery I would probably send our kids to private school, but we just don't have that kind of money especially if we are going to help with college. One thing I have noticed is that my private school friends complain just as much about their private schools as my public school friends do about public schools.... so there is no Nirvana.
Check around, visit schools and it will eventually become clear.
Good luck!