They've not passed that many previously. And why if a child has passed the first test would he voluntarily withdraw before the second stage? I imagine there'd be very few reasons why. Even if the school was not the very first choice, I think you'd want to see the process through so as to have it as a possible back up. I can surmise that some people heard about the 781 Wallington passes and took fright
I should have explained it as follows: In previous years at the end of the SET you would have had the news that your child had passed and would now be able to sit the second stage for Sutton school. However, this year many people got a mixed message you passed the exam but 781 boys did better than you. I know people say that the how the exams are weighted makes this of less import in the eventual ranking, but emotionally it is very different from the response to just knowing it was a pass. There's an American academic Kahneman 'Thinking,fast,and slow' who has written some interesting stuff on emotion risk and chance. Basically says that emotion distorts response to the same level of statistical risk or opportunity.
My view is that announcing how many boys passed the Wallington test - especially given that the number was higher than had got through to stage 2 previous years - likely had an unhelpful affect on many candidates.