Why Can't Teachers Write?

Bill Hennessy writes (link in title) about the poor quality of a letter received from his child's principal. I think most people with children in school have encountered this problem. Of course, everyone makes mistakes (there are probably some in this article), but the level and frequency of errors found in communications from teachers is a bit of a concern. This is professional correspondence, and the quality of writing should reflect that fact. In addition, these teachers are entrusted with developing the writing skills of our children; therefore, an expectation that they demonstrate good writing skills is not out of order.
I have occasionally corrected letters from teachers (the "sign and return" types) and included a note regarding my concern about the language quality of the letter. I have never received a "thank-you" for my efforts. The most irritating problem I find is the use of plural pronouns where singular pronouns are required. For example, a field-trip permission slip included the sentence, "Your child should bring their lunch." Obviously, the language is falling prey to PC in an attempt to use non-sexist language. What irritates me most is that the error is so unnecessary, since it can easily be fixed by saying "The children should bring their lunches," or "Your child should bring his or her lunch." The latter is probably deemed "awkward," but surely it is not as awkward as the blatant grammatical error?
I think we may lose the battle for singular pronouns in the end, since Zondervan, in its latest revision of the Bible (TNIV), has embraced the use of "they" and "their" as singular pronouns in its fanatical, over-the-edge, who-cares-what-the-original-intent-is subversion of the English language and the Christian religion. Historically, as the Bible goes, so goes the language.
All of this would have been prevented, had the old standards of the normal school, where teachers were expected to be able to speak and write with impeccable grammar, been upheld. But the modern (or is it post-modern?) ed schools have long since renounced any such standards. English and math are out, self-esteem and problem-solving are in.