I don't think there is any such thing as a flawless English piece, but I think if you get full marks on a SAC or an exam it just means it is the piece that is the CLOSEST to being flawless.
The closet thing to flawless is not flawless.
Consequently meaning that a perfect 100/100 is not deserving.
I truly don't understand why some English teachers are so slack with marking. At the end of the day, your students SACs marks will just be bought down.
When you have roughly 10 50's in a school for English, I don't think you can say that English teachers are so slack with marking. Obviously a student will not write a perfect piece, so to offer a 10 for an essay does not signify perfection, rather, you are so far ahead of the average that it can be considered relatively perfect.
But I do acknowledge that in some schools, there are teachers who mark particularly easily for the student's level. This false marking is does not augur well for good exam results.