Hmmmmm
As an ex teacher I can't put my hand on my heart and swear I never made a spelling or punctuation error in the sometimes lengthy feedback I used to leave on students' work (I taught in FE so was marking GSCE and A-level essays).
But you know if I'd been guilty of habitual apostrophe crimes I would have been profoundly grateful if someone had gently pointed it out to me and shamed me into dealing with it. Kids lose confidence in teachers who do this sort of thing on a regular basis......
Last year my 7 year old daughter's class teacher was a 23 year old NQT. Very nice, very hard working. But I found some basic punctuation errors in her display work that she'd pinned up as a reference aid for the children. I took her aside and quietly pointed it out - made sure I did it out of hearing of the classroom helper, the children and the other parents. I would have hated her to have lost face or felt humiliated, but then on the other hand there was not way I was going to have her confusing my daughter about these things either....