Key Stage 2 covers ages 7-11 and school years 3 to 6.
You may hear teachers or other parents split this stage into
‘Lower Key Stage 2’
and ‘Upper Key Stage 2’.
This is simply a way of segmenting years 3 and 4 (lower Key Stage 2) and years 5 and 6 (upper Key Stage 2) so that progress can be monitored within context. Children will take part in the second set of national curriculum tests (SATs) at the end of Key Stage 2, with the results helping to focus their learning going into Key Stage 3.
Source Reference: What are the Key Stages in the National Curriculum? - From WHSmith