Of course teachers should be sound on spelling, grammar, punctuation and numeracy, though I'd be prepared to cut an otherwise competent chemistry teacher, say, some slack if her spelling was a bit dodgy but she knew the symbol for nitric acid and didn't spell it sulphuric acid.
But as always, I am amused by posters who bang on about the importance of SPAG, while making basic errors that can't really be explained by just mucking about.
If you know the basics, they flow automatically. Though not impossible, it's quite hard for someone who does to consistently put apostrophes, for instance, in the wrong place.
There will be mistakes in my post, particularly in my use of commas which I'm weak on. The difference is, I don't claim that it's easy or make unconvincing excuses for my mistakes.