bull
You haven't a clue.
"if a child is not putting in their best effort, I will make them do their best even if that means starting again."
Doing your best in art means never starting again unless you document it. That's not my opinion, it's the national curriculum's.
I am sure you are an excellent teacher, high expectations and all. This does not make you a good art teacher. In fact, I think it is virtually impossible for a primary school teacher to be a good art teacher unless they are art trained themselves.
"You may have written a thesis on it, but if a child is mucking about, being silly and chatting in a lesson and then produces a below-par piece of work then it is pretty obvious to anyone, not just the teacher, that they have not put much effort into it. That's not subjective opinion; that's observation of the facts."
There are no facts in what you have just written- it is pure conjecture. I sincerely hope you do not write any official documentation about children's behaviour in this manner.
Imagine a child who is abused at home, has had no breakfast and has to wake himself up to get to school. He has missed the bus and walked in. He was late, he got told off. The other kids told him he smells. he is now 'mucking about', 'being silly' and 'chatting' in class as it is making the other kids laugh and he likes the attention. He is starving hungry, cannot see the point in the work, yet he does a drawing. Not put much effort in? Are you having a laugh, or do you just not teach these children?
"It is not doing them any favours to make excuses for poor behaviour."
These are reasons, not 'excuses'. This tells me you have little experience of teaching children with serious behavioural difficulties. Unfortunately, many mainstream teachers share your view.
"If you think you are better at judging situations I suggest you walk a mile in a teacher's shoes, rather than sit on the sidelines sniping at their inadequate, (in your eyes) subjective opinions."
Err... have you read any of my posts? I am a teacher, and have been for many years. Does that count?