I'll stab a guess here - I think they assess at 4+ to filter out any children with major learning difficulties. There are children with learning difficulties such as dyslexia etc in school but the broad spectrum is regular kids/regular learning abilities.
Both my children had to be assessed - we joined last September when DS was entering Yr4 and DD was entering Yr2.
DS sat verbal, non-verbal reasoning and maths tests. He had to read to the Head as well.
DD just spent a day with the relevant class and the teacher heard her read and watched how she joined in etc.
Over-subscribed ....hmm....I'd dispute that! One thing I have noticed is SGS is very transient - lots of kids come and go throughout the year, in all age groups. Loads of children from different countries being moved about with parents jobs etc. In the lower years, there are two forms per year, each with about 20 kids and there are places for new children to join. I believe there is an initial mad scramble for nursery/reception places but people drop out for various reasons and there is space in there!
In year 3 (age 7+) they have a fresh intake and they go from two classes to three classes, each with about 15 children. Once again, this number doesn't remain fixed because of the transient nature of the school. My very cynical mind has wondered, briefly, if school are quite happy to take children who pass their assessment but with less than outstanding results, just to fill seats.
I am very cynical though!
SGS seem to like 'all rounders' but are happy with 'academics' too. There's such a big range of clubs and sports on offer, as well as the regular school stuff, ALL children can find their place. I think CHS is like this too - certainly from what I've heard. I'm talking about, rugby, footie, science club, art club, cricket, swimming, music club, choir, speech & drama club....it's a right mixed bag!
One down side - the feckin' carpark drives me to distraction (and tears). My two use one of the excellent school buses thank Gawd but I do have occasion to go down to school for pick-up and drop off sometimes and I HATE IT!!!!!!!!!
On the plus-side - the other parents are seem dead normal. I was worried about meeting a load of Hooray Henrietta's but most are nothing of the sort!