I was with (via Zoom) twelve Y6 teachers from different schools on Friday. You are not alone.
Although each class is obviously unique, the general feeling was that reading had held up for those who had done anything through lockdown.
Fluency was slowly but visibly returning in arithmetic through daily work, and gaps had been identified and a plan was in place to try to get them back on track.
But writing! A lack of stamina, lots of basics (handwriting, punctuation, spelling, sentence construction) below where they were before lockdown.
So, firstly, you are not alone.
Secondly, this is not yet over. Two teachers were currently isolating as their bubbles have popped and everyone knows that could be just one positive test away for them too. So it is not a calm, smooth route to catch up.
HOWEVER Y6 teachers (or teachers of any yeargroups) can only ever take children from where they are now, as far as they can go. Just because there are end of KS2 national expectations, and expectations of the progress these children will have made KS1-KS2, this situation is unprecedented and we cannot expect the same outcomes.
The mental health of both children and their teachers needs guarding and it is critical that steps are not skipped in a race to targets. We need to rebuild step by step, ensuring we build or rebuild on firm foundations each time, or the learning is likely to fall away through short absences or when further new teaching is given.
Children and their teachers need to feel successful, and need to take time to recognise progress, and celebrate it. So if a child only managed to produce a paragraph today, tomorrow they need to write at least an extra line = progress. If they had forgotten how to spell 'said' yesterday, today they need to spell it correctly = progress. It isn't the progress we are used to in Y6, and it often isn't from the same starting points, but it gives us the start on which we can build.
Slow and steady. Be kind to yourself and to the children. No one expected to be teaching or learning in the time of coronavirus. The fact that you are doing it and the children are recovering their brains and attitudes for learning is progress. Keep up the good work.