I will let one of Scout's classmates take a stab at answering your question first,
“He’s one of the Ewells, ma’am,” and I wondered if this explanation would be as unsuccessful as my attempt. But Miss Caroline seemed willing to listen. “Whole school’s full of ‘em. They come first day every year and then leave. The truant lady gets ’em here ‘cause she threatens ’em with the sheriff, but she’s give up tryin‘ to hold ’em. She reckons she’s carried out the law just gettin‘ their names on the roll and runnin’ ‘em here the first day. You’re supposed to mark ’em absent the rest of the year…”
Atticus later tells Scout that there is more to this story than what she thinks. Scout wants to quit school, knowing that she is smarter than Burris Ewell. Atticus takes time to explain to her what she needs for instructional and social development. However, the Ewell family does not have the same values. So, the government officials and the truancy officer uphold their duty to the law to get the kids into school, but the parent really has to make them go. This is obviously something they have struggled with regarding Bob Ewell. They are allowed special privileges because Mr. Ewell won't do anything about it and no one wants to fight with him. I don't really think we can call these special privileges, they are more like disadvantages.
The Ewells are allowed special privilages because nobody wants anything to do with them. The people of Maycomb are embarrased and discusted of the Ewells. They give them what they are obliged to do and leave them alone.