After the Reception class in a primary school, children are taught according to the National Curriculum.
The National Curriculum is separated into Key Stages -
Key Stage 1 is for children aged 5-7 (Years 1 and 2),
KS2 is for children aged 7-11 (Years 3,4,5 and 6),
KS3 ages 11-14 (Years 7,8 and 9 at secondary school) and
KS4 ages 14-16 (Years 10 and 11).
Try
http://www.qca.org.uk/qca_7120.aspx for more info.