Most state schools start at year 7. Some areas have primary schools followed by middle school, students at middle school would stay on till they are 12. Most private girls schools are only upto year 6, as most private girls secondary schools start at year 7. However, boys private schools have the option of them staying on till 13. Some boys private schools start at year 7, but others(like Harrow/Eton) only take them when they are 13, hence them staying on longer at primary school.
It varies most areas transfer in state sector from primary to secondary at 11(end of Year 6), but some others may be a year later. Infants to Juniors is end of Year 2, aged 7+. Private schools differ - Pre Prep to Prep at end of Year 2 or occasionally Year 3 , Prep to Secondary end of Year 8 (aged 13) although many private Secondary schools(usually girls or mixed) also take them from 11 now, especially if they have their own Prep Department feeding through. hth
Pretty much what everyone else has said. Secondary starts at 11 (yr7) except for some private schools start at 13 (yr9) and some areas who still have the old middle school system but this is being phased out in alot of places.