For openness I am a secondary school Data and assessment manager
The level 6 is in my opinion a waste of time at primary and results in cramming and poor practices in many primary schools desperate to tick the box on their OFSTED questionnaire.
It was introduced last year when my child was year 6, and badly managed by his school with booster classes after school that they all hated. Not one of them got a level 6 and I know of 2 highly achieving maths children who didn't even get their level 5 because they were so confused.
Luckily it's all moot at secondary, none of the schools they have gone to, nor the one I work at, pay much attention to KS2 SATs - we base test and assess them on entry, set them loosely, re-set and re-target over first terms and have fluid sets over KS3
So do not panic. We normally see our english and maths teachers working furiously in term 1 of year 7 to get students back to KS2 SATs results as they tend to drop over summer and as a result of teaching to test practices or on the flip side see students soaring as they are taught appropriately at secondary level
(On a personal level my child achieved a high 5 in maths at KS2, didnt get level 6 so he felt like he'd failed no matter what we said and within 6 weeks at secondary -not the one I work at - his end of first term data report showed him at a 7c)