You can introduce curriculum content earlier or later for some pupils
This is because within each Key Stage (KS), you’re only required to teach the relevant programme of study by the end of the Key Stage, rather than a specific year.
For example, if a year 3 pupil in a mixed-age class is working beyond the year 3 curriculum, their teacher can introduce them to year 4 curriculum content.
You can also teach Key Stage content in an earlier Key Stage, if appropriate.
This applies for English, maths and science (even though these programmes of study are set out by year) - see pages 16, 100 and 145 of the National Curriculum framework for KS1 and KS2. The programmes of study for foundation subjects are already set out by Key Stage.
Make sure teachers are clear on the age-related expectations for the divided year