At best, it's a typo on a sheet produced by someone else that the teacher has missed...
Then, next best, its an error (I do it sometimes and then correct myself, in my accent 'would've' sounds like 'would of' even though I know it isn't)... and should have been spotted and corrected.
At worst, the teacher thinks it is correct.
If that were a pupil who made any one of those errors, they would rightly be pulled up on it and corrected, and either told off for not checking their work properly or the incorrect use of 'of' instead of 'have' would have been addressed.
Why would it be any different for a teacher?
If a kid wouldn't have the excuse 'Oh Miss I was busy and missed it because I was tired...' then nor should the teacher.
Yes, I would be that parent and I absolutely was that kid who would correct worksheets and hand them back to the teacher (in red pen, because thats how stuff is corrected!).
I also had teachers correct me on my use of words they didn't know and it infuriated me no end - amused to see someone else had 'tack' (as in horse tack) corrected to 'tackle' - I had 'bridle' changed to 'bridal' when the context was indeed horsey, not wedding-y.
I had a stand up row with a geography teacher when he asked us to name cave formations... expecting 'stalactite, stalagmite' and I added 'helictite and heligmite' and he crossed those out of my book, so I raised it in class when he went through it again.
Teacher: What do stalactites and stalagmites do?
Kid: They grow down from the ceiling and up from the floor sir
Me: WHat about helictites and heligmites sir?
Teacher: 'I told you to stop pissing about, what do THEY do then Diddlin'?'
Me: They grow in all directions, round and round sometimes Sir!
Teacher: Get out!
Being the ultimate smart arse (it really is no wonder teachers did not like me), I brought a book to the next lesson with photos of both (and straws, flowstones, rimstone dams, columns, popcorn, bacon, drapes)... written by my DF. I did get a somewhat forced apology for that.