Key Stage 1 (Wikipedia)
"Key Stage 1 is the legal term for the two years of schooling in maintained schools in England and Wales normally known as Year 1 and Year 2, when pupils are aged between 5 and 7.
This Key Stage normally covers pupils during infant school, although in some cases this might form part of a first or primary school. It is also the label used for the third and fourth years of primary education in Northern Ireland. In Hong Kong, it is used to describe Primary One to Primary Three."