In Chapter 1-3 Why didn't the Ewells have to go to school?

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.