Probably one of the very worst things about the English writing system (and it has a huge long list of bad things about it) is that it very clearly employs 27 letters in the spelling of words but there is a huge and long-standing conspiracy to market it as having only 26. Insane, but that's what English has done.
The 27 letters are ASCII octal 101 to 132 (the upper-case letters), 141 to 172 (the lower-case letters), and 047 (the forgotten letter).
The punctuation marks are an entirely separate disaster from all of this: the ASCII characters with octal numbers 041, 042, 050, 051, 054, 055, 056, 072, 073, and 077 in some cases have two distinct typographical shapes in print depending on context, one of them has a shape identical to one of the letters, and so on, a terrible mess. There are other characters that appear in written English, but they are unimportant, mostly abbreviations or special spellings for certain words (& for and, % for percent, $ for dollars, and so on), and there are a few other marks and symbols. But the forgotten letter, octal 047 (which causes such a mess for other reasons that it has to have two Unicode numbers, 0027 and 2019), is not a punctuation mark — though it is typographically identical to one of the punctuation marks); nor is it a word abbreviation, or any kind of a textual decoration. It is a letter that you need simply for spelling quite elementary word forms.
One of the worst things about the forgotten letter is that it never stands for a sound in native English words. Indeed, it could be argued that it never appears as a letter within the plain form of any lexeme, and never occurs initially in any word in modern English. But it does appear as the first letter of the two-letter genitive singular suffix of regular nouns; as the second letter of the two-letter genitive plural suffix; as the middle of the three letters that spell the suffix identifying the negative form of auxiliary verbs; as the first letter in the written clitic forms of am, are, had, has, have, is, will, and would; and it has miscellaneous other uses. But though obligatory where it occurs, it never corresponds to any sound in native words. (It sometimes corresponds to the glottal stop in foreign loanwords.)
So don't be too harsh in blaming people who can't spell English. It is tough for them — they will get blamed for their failure, and they will attract the mock ire of Lynne Truss; but it does not mean they have low intelligence. It certainly is not their fault that the alphabet is such a mess that the very number of letters has traditionally been misreported, and is still misreported today.