Question Author
Thank you both for taking the time to answer.
I did read about the concept of standardised scores before I posted the question, but your explanation is simpler and easier to follow than the online explanations that have been puzzling me and I think I understand it a bit better having read your account of it.
My understanding now is that 109 is not a score, but is the place that the child occupies in the full list of standardised scores, where the "average" child occupies place number 100.
I suppose therefore, whether child in place 109 gets into grammar school depends how many other children occupy the same or a higher place in the list of standardised scores.
That's what I think is the case, based on my interpretation of Buenchico's explanation, but if I'm wrong I hope somebody can put me right.
My brain hurts trying to understand it.