I know of 2 people who were illiterate before they started flying training, both learned to read a write to do the exams. One did the old restricted licence the other went on to a commercial career. I myself left school at 16, did a trade then started flying training at 21, have been flying for a living since. Bad schooling is no excuse, if you want something bad enough then you put in the yards. The cost and availability of training is the main impediment now. In the "olden days" every airfield had a family training organisation, also there was constant GA traffic everything from charter, freight, AG and feeder airlines. Now it's very unusual to see a GA aircraft. Shortly CAA will reach it's goal of having no aircraft flying.