The lack of realistic places in schools, paired with the overpopulated cities in the UK.
There is a very, VERY old system in place to give kids the right to be in one school or another, and the selection is done early as (like Karen pointed out) there are a lot of appeals.
So, they give you a form, much much earlier in the year,
you fill in, up to 5 schools choice and send, then you can go to what they call "open morning/afternoon/evening , in which headmistresses and other form tutors speak and explain why their school should appeal to you as a parent, to the child as a student.
Kids here get into secondary school at 11 years old, which I think is too young for such an abrupt change, they also are, in a way, forced to choose what they want to do later in life, as secondaries here are divided into specialities that will give them more of one subject or another...
I was lucky enough that Sarita knew what she wanted to do since she was 8, but I saw many of her friends collapse because of this early choosing....I think the index of students who change carriers even within the first 2 years of Uni is very high here because of the system in place for secondary...but hey, am talking too much again....
soon you all gonna wish I wasn't in such good health to spend time in the forum