@AnneOfCleavage
We had a tricky year 1 class last year at my school. The main thing is to be firm and prepared to wait for quiet, or lining up sensibly. Give everything extra time to install this discipline. Set off for assembly early and be prepared to go back to the classroom and line up again etc.
Our year 1 teacher put each table into teams and gave out table points for good behaviour. It soon focuses the mind when children are getting a table point for sitting nicely, tidying up, giving a good answer, helping another child. Praise, praise, praise the good behaviour and the other children will want some praise and reward too. Rewards can be extra playtime, taking a Teddy home, watering the plants, a sticker, a trip to the headteachers office for a well done, name on a super star board.
I made a catch-me-if-you-can poster of some of the good behaviour I expected. A photo of someone sitting nicely on the carpet, someone with their hand up, someone lining up nicely. I would then say, “well done Lucy, Aaron and Amol, I have caught you sitting nicely, go and get a table points. Oh David and Jaz I have caught you sitting nicely too. You may get a table point.” Then start the lesson!
It is exhausting and takes time. Good luck, a day at a time…