Spelling is obviously not the most important thing learnt at school, but it does have a useful function in that correct spelling along with correct punctuation does help to avoid misunderstandings.
However, spelling like grammar does change from generation to generation.
I was for instance taught that the present participle when used as a noun should always be preceded by a possessive pronoun, not by a pronoun i.e. the sentence " My parents object to me staying out late in the evening" is incorrect, it should be "to my staying out"
I doubt that any youngster would dream of saying or writing "to our swearing" they would quite naturally say and write "to us swearing".
Likewise I expect the apostrophe will disappear from written English in a generation or two, as we seem to be the last people to know the rules governing its use.
The spelling checker and grammar checker are useful, but not infallible - however much I insist that I want British English, mine reverts to American English, and I constantly have to insert "u" in colour, favour, etc. or a second "l" into travelling! Even if you can get it to respect the form of English you use, it does not necessarily know every word that you know.
This said, I agree, school teachers should be able to spell, use punctuation correctly and know the multiplication tables, plus a vast lot of other things like when to use which past tense in French ( I once invigilated a written A level exam where there was a sentence that not even native French speakers were confident about translating) or explain the difference between strong and weak German verbs, but we are none of us perfect, after all.
Secretaries are just as guilty as teachers. I once received a letter addressed to Mr. followed by my Christian name Elisabeth and my surname. As it was for a gynaelogical appointment, I was considerably miffed!