It led to this photo:
What’s so wonderful about this is that every single apostrophe is wrong even in the words which should have one. That takes some doing. It’s slightly disappointing that there are two S’s and an s which don’t have apostrophes, but every s at the end of a word has one. 
Given this level of confusion, part of me is wondering whether one day something like this will happen:
Since the letter s is always used with an apostrophe, the apostrophe becomes part of the letter. First there are two versions, depending on the placement of the apostrophe, but by this stage nobody can remember which is right anyway and it’s such a pain trying to choose between the two different s’s that the two forms merge.
Actually I suspect this won’t happen, since so much English is now typed rather than handwritten. More likely the apostrophe will go out of use unless the very straightforward rules for its use become widely known again. It’s an interesting thought, though. And drawing the illustration gave me an excuse to experiment a bit more with the graphics tablet that arrived the other day.
 S’s as the plural of S is correct: letters of the alphabet are the sole exception to the rule that apostrophes never make plurals. Consider the other ways of doing it and you can see why. With no punctuation, the plurals of a, i and u would be as, is and us, which are virtually impossible not to read as the standard two-letter words. Try giving the a, i and u italics: as, is, us—almost as bad. Or putting them in single or double quotes: “a”s, “i”s, “u”s, ‘a’s, ‘i’s, ‘u’s. It’s hideously messy and looks rather strange. The simplest solution is a’s, i’s and u’s. If you regard the letters as being short for their spoken names, the apostrophe becomes logical for some letters: t’s is short for tees and z’s is short for zeds for example, with the apostrophe representing the omitted -ee- and -ed-.