I'm sure David does his fair share of educating the older kids. Prissy is barely literate.
eta I'm worried that David is more concerned with bible memorization than real education. But even so, I'm sure he notices that Paul and Davia's basics may be lagging behind other kids at church. For whatever reason, the Wallers as a whole are better educated than most of these fundies that we discuss, and he may be stepping in for a reading lesson (probably the bible), or maybe even math.