Simon Briscoe in the FT on the effect of birth date on acceptance to grammar schools (I take it the point is that a summer child is assessed early, while an autumn child, born when the school year begins, has a whole year's advantage on someone born a month or two earlier, but this is not spelled out and frankly your guess is as good as mine)
How do grammar schools select children?
The short answer, according to the national school admissions code, is by "ability". Their way of assessing ability is mainly by a combination of English, maths and reasoning tests. A minority use a head teacher's report as well, for borderline cases. Sometimes individual schools select, and sometimes local authorities or groups of schools.
What is going wrong?
Some grammars and local authorities are selecting a heavily disproportionate number of children born early in the school year. In the most extreme cases there are twice as many of the older children compared with the younger ones. The dominance of older children vis-à-vis younger ones follows a progressive pattern over the year. At every month after September, a child's chance of getting into the grammar sector declines. This could mean that some able summer-born children are not getting into grammar school.
Why is this happening?
Earlier research shows the average September-born child performs better than the average August-born child in school exams - possibly because they have had more months to mature, and perhaps because they have a psychological and physical superiority over their classmates at the beginning of their school days that hardens into academic superiority. Many grammars and authorities respond to this by adjusting their results for age - pushing the marks up for younger children and down for older children. Theoretically this should eliminate autumn preponderance. But there are two possible flaws to this - that schools may not be adjusting enough for age when marking exams, or that they are not accounting for age when considering head teachers' reports. It may be that schools do not know that their admissions are in effect age-biased. Alternatively, some may know they are, but are deliberately selecting the older children.