By law, schools must report their test and teacher assessment statistics to parents along with a national comparison. Here are the templated/materials provided for this purpose this year - note that the test tables do not contain a column to record level 6, because level 6 is not measured. Note that the teacher assessment tables do - to allow the school to record their level 6 data. Nationally, that data is recorded as 0%, because it refers to exceptionally able children and the statistics would be less than 0.5%. But it exists.
Sounds like your ds should be part of this data. An obligation which his school seem to have side-stepped neatly.
It isn't arrogance to know that this is factually correct, or where to find evidence to back it up. How can level 6 not exist, according to one SIP, if:
a) The ARA refers to tasks teachers may use to back up their assessment, with a separate website provided for this.
b) Several teachers on MN have children assessed as level 6 at their school, either now or in the past (Spanieleyes, Mrz, etc).
c)A table provided by QCDA has a column specifically for that purpose in teacher assessment, but not in test results.
I know that 2 levels is of prime importance. But if you were to work in a school, and see what else you are judged on, besides this (there are many, many more sticks to beat us with, even if the two levels progress is at the top) you would see that so is teacher assessment, as acknowledged and inspected, and commented extensively on, by OFSTED and SIPS. I'm going to assume that you can't possibly know this, since your role is simple test dat, rather than accuse you of arrogance or ignorance, since I am a little more polite than you are, it would seem. 