I have no familiarity with this show, but major urban areas in the South, Atlanta most of all, have seen huge migration from other parts of the country, especially the Midwest, especially middle/professional-class transplants. That means there are large numbers of people in Atlanta and other southern cities who do not have Southern accents, and whose children either barely have a Southern accent or don't have one at all. So it's quite plausible that depending on the neighborhood, there could be whole schools full of kids that don't have Southern accents.
Anecdotally, when browsing lay discussions of American accents on reddit, I have seen people who live in Atlanta - either transplants or children of transplants, I would presume - express disbelief that "anyone actually talks like that anymore" or claim that "nobody talks like a hick here", that is, in reference to an ordinary Southern accent. So I think it's very much possible depending on your location, profession, social class, social network, etc, to live in Atlanta and have a peer group where no one has a Southern accent.
I have relatives who moved from the Midwest to Atlanta 20 years ago and they and their children still don't have Southern accents.