Here's the advice from the 2011 ARA, which legally all state schools must follow.
It states in paragraph 4.2:
Pupils working above or below the levels covered by the tests
QCDA has produced a suite of optional tasks covering levels above and below the test levels. The optional tasks provide additional evidence that can contribute to teachers? assessment of their pupils. Optional tasks for pupils working above or below the level of the tests are available at www.mycurriculum.com.
The tests cover levels 3-5, but a teacher assesssment is also required in Y6 with 50%/50% equal weighting. The above advice is to help teachers who may have children working at level 2 or level 6.
Later on in the document, in Reporting and using pupils? results,
where the document refers to submitting teacher assessment data, it also states:
Teacher assessment codes:
1?6 Curriculum level achieved (etc).
So, up until last year, it was possible to teacher assess at level 6. This year, there are externally marked level 6 tests for the first time in 9 years, and they are scheduled to take place the Monday after SAT week.
I've heard (only on MN) that at least one LEA does not acknowledge level 6 at KS2 - I have no idea how they have got away with it for so long, since the statutory info in the ARA is very clear. I hope they will change their practices now that level 6s can be included in the test data again (and should never have been legally inadmissable as a teacher assessment).