We cannot assume that all teachers have a 'good grounding in English.' If they are literacy specialists then yes, but your average 'other' subject specialist in secondary /multi-tasker in primary doesn't have too much expected of them to meet entry requirements for training.
I have been a lead literacy teacher in primary education and am aware that many teachers are under confident about grammar technicalities and spelling rules. Working to degree standard in anything, there is always spell checker and in my experience, university tutors will not correct spelling and grammatical errors.
From within the profession, I have been surprised at times at how some teachers are daunted about having to write a report, or a letter, but that is also actually quite common.
BUT all messages going home to parents or on the school website should be checked.
As for teachers making mistakes on Gransnet - well, perhaps like me, others view this as a leisure activity and therefore don't fuss over every phrase, dot or comma.