Why wasn't Jennings forced to attend school?

There wasn't enough room for everyone, thus, children who won't be around very long, or children who are slated to be placed are not forced to go to school.

“What’s a part-timer?” I asked. “And why don’t they go to school?”

“They got no room for everybody. A part-timer ain’t gonna be here all that long. You’ll either go home or be lent out to live with somebody.” He added quickly, “Don’t go bawling on me, now. It don’t mean you will be lent out. I was just using that as a for-instance.”