Hi Guddy - the type of school your child will go to depends on what part of the country you live in. I live in West Midlands and we have an Infant/Junior then on to Secondary school system. Child joins infant school in the September following their 4th birthday. The cut off date for intake is 1st September so if child A were born on 31st August 2000 and child B were born on 14th September 2000 they would be in different school years, child B going the following year. (NB. Some areas have a January or Easter intake too). The school years are Reception, year 1, 2, etc up to year 11. Infants includes Reception-Year 2, Juniors includes year 3-6, and Secondary includes years 7-11. Then come the key stages Foundation, 1, 2 & 3.
Foundation Key Stage is Nursery (optional) and Reception.
Key Stage 1 - Years 1 & 2
Key Stage 2 - Years 3,4,5,6 (the Juniors)
Key Stage 3 - Years 7,8,9
Then GCSE's are taken in year 11.
Of course some rural schools have a very small number of children and ages doesn't appear to have too much bearing as quite often children spanning a few years are taught in the same class. A rural mummy may be able to explain this better.
Also in other areas you have the Infant/Middle/High school system which I know nothing about at all other than it doesn't follow Infant/Junior/Secondary movement but the curriculum is the same.
Comprehensive schools are secondary state schools and are available to everyone; grammar schools usually need an entrance exam pass to attend.
Hope you understood this but there are lots of teachers on this site who will be able to help further, Popsycal teachers juniors and Hulababy teachers secondary children to name just 2.