You can find the answer to this in Chapter 3.
Officially, the Ewell kids have to go to school just like everybody else does. But they never really do. We are not told exactly why, but it is pretty clear that it is just because they are all around pretty nasty people.
We are told that the truant officer always goes and gets them to come on the first day of school by threatening to get the sheriff. But after that, she just figures she has done enough and lets them stay home.