One of my teachers at school was in the last year to do National Service. He'd just finished reading maths at Cambridge, so he was made a Sergeant Instructor teaching remedial squaddies to read and write. He was given six weeks to do it and told that he'd be in trouble himself if they didn't learn.
Although he was horrified initially, he said that it turned out to be quite easy. The first discipline problem was dealt with by having the lad run round and round the parade ground in the pouring rain. A corporal pointed out that if they were literate, then they could read porn - of which there was a more than adequate supply on the base. The lads had nothing else to do and got on with learning with considerable enthusiasm, and nearly all of them got up to the appropriate standard in a fortnight.