School are split like this:
Foundation Phase (FP) covers Nursery to Year 2
Sometimes FP is used to refer to Nursery and Reception and Key Stage 1 (KS1) to Years 1-2
Key Stage 2 (KS2) refers to Year 3-Year 6
Key Stage 3 (KS3) refers to Years 7-9 (at the Comprehensive)
Key Stage 4 (KS4) refers to Years 10-11 (GCSE years at the Comprehensive)