There is a good reason why born-in witlesses tend to be crappy in spelling (and just about everything else). Just try going to the Kingdumb Hell one Thursday evening and see how late the boasting session lets out--now, when does that leave children time to study for school? And they are tired all the time (and it is going to be even worse when you factor in the field circus arrangements for 3:00 in the afternoon for school children--even less time for studying for school. It is a well documented fact that children that do not get enough sleep tend to learn poorly (and anyone can do the research on that subject at will, using any sources one wants).
Those of us who went in as adults already learned to spell (and all their other school subjects) while they were worldly people. Thus, they have a better chance of learning to spell than those who had no chances outside boasting sessions and field circus. And that is not very likely to deteriorate--as long as they are able to find the words in their Bibles or the Washtowels, they will continue to be able to spell them correctly.