I think there’s two things here.
The first is that parents do need to know where their child sits in relation to expectations for age. Many parents of children with additional needs are not aware of what the expectation is at their child’s age and so are not aware of how their child is affected by their difficulty – whether they under or over-play the difficulty. So a measure of attainment against the rest of the class is useful IF there is a comparison eg “10% of the class achieved grade A, 30% Grade B, 40% Grade C, 20% Grade D”
However this isn’t so useful if the child’s attainment is wildly out of line with the class as the grade probably covers quite a wide range at the lower end.
The second thing is that some SEN learners will be stuck on D forever no matter how hard they try. This is discouraging for all. Even in the reporting used by many UK schools now – on track / not on track – being not in track your whole academic career doesn’t describe your progress.
For this reason many schools use a small steps assessment tool.for SEN learners that measures progress not attainment. Sone reports detail what new skills the pupil has gained, and outlines the next skills they will be working on..
A separate grade for effort may also be helpful.