You'd probably be shocked how many illiterate people there are. Although I suppose a teacher would not be. Most people who are actually illiterate try to find ways of people not finding out and getting round it. An acquaintance (well, two of them actually!) of mine for example pretends that he 'can't be bothered' to read his text messages and asks a trusted friend to reply to them. In reality this just covers for the fact they can't actually read at any kind of realistic adult level. Regardless of numbers, these people exist, and they're part of the 'class' of people you like to feel better than.