How do head teachers decide which teacher gets which class?

[GiraffesBaby]

Is it all quite complicated?

My DS's old primary school seemed to keep Yr R, Yr 2 and Yr 6 teachers the same. I suppose they are a bit more specialist years with EYFS and SATS

All other years mixed around a fair bit

@JoyOrbison

Our school was:

Weakest class needed strongest teachers

Sat's classes stronger teacher

Eyfs usually same member to ensure established experience brought out best in pupils starting school

So years 1,3,4 and 5 were often most likely to see change unless there were issues with a particular cohort that needed a strong lead, eg one year a class had gone through 8 temp teachers and the head having to teach in one year. They were a tricky class so they were needing a confident no nonsense teacher.

Year 1 is now the most important year in the school in an Ofsted inspection

You cant have a lemon in Year 1. Year 3 is probably next.

Normally preferences are noted, with it being made clear that this doesn’t guarantee a choice. It’s worth not annoying decent teachers by repeatedly placing them in year groups they don’t want to be in. Year 2 is often an unpopular one (SATs, small people) and required negotiations and rotating staff a bit if you don’t want Easter resignations.
Beyond that it’s a balancing act of experience, sustainability in the school (training new teachers), factoring in special needs/ classes with need for support etc.
Usually teachers are spoken to individually before it’s announced.

High schools are usually organised in departments, supervised by the Head of Department.

I’ve had some not great experiences though, outside the norm. I started in secondary teaching then applied for a year 5 teacher job.
On the inset date in September I was shown to my room and it had tiny seats, the Head just said ‘oh, changed you to year one’ and walked out!. One day’s notice. I was not the best year one teacher...
I changed schools quickly for a year 6 class, did that a few years, moved to year 3, then I went p/t for a year after a baby so was given nursery that year (omg...). Went f/t and got year 6 back.
It can be just plugging holes in reality.