Secondary schools admit children from the ages of 11 through to 16. This is known as Key Stages 3 and 4. If the school has a Sixth form, children will stay there until they are 18.
The year groups at Secondary School level are:

Year 7 (age 11 – 12)
Year 8 (age 12 – 13)
Year 9 (age 13 – 14) SATs testing takes place (end of Key Stage 3)
Year 10 (age 14 – 15)
Year 11 (age 15 – 16) The year most students take their GCSE exams (end of Key Stage 4)
Year 12 (age 16 – 17) AS level exams taken in this year (also known as the Lower Sixth)
Year 13 (age 17 – 18) A-Level exams taken in this year (also known as the Upper Sixth)

Starting at a Secondary school is probably the biggest change a child will face in his/her school career. Parents will be expected to pay out for a new school uniform and other equipment. Children will almost certainly move from a primary school of just a few hundred pupils to a secondary school of a thousand or more pupils. All secondary schools have induction days at the end of the summer term where new pupils entering year 7 can visit the school and meet their teachers and learn where everything is. If parents have any concerns about their child’s welfare at secondary school they should in the first instance speak to their child’s form tutor, followed by the head of year and finally the deputy head or head teacher.