I disagree that 3 years of needing to run a bulge class is always because of a big fuck up in admissions. It might not be this at all, but precisely due to the need to honour offers under the traditional system.
So schools look at past patterns of acceptance rates to offers given when deciding how many offers to make. Some schools see more patterns than others. When there are lots is similarly ranked schools in a small geographical distance, there are less likely to be patterns that can easily be used to base offers on. As the no,s of schools families apply to have increased, these patterns become even less easy to see or don't even exist as the no,s who have a given school as their first choice vary significantly year to year.
So it's perfectly possible to have 1 year when you are a bit cautious with your offers and don't fill. So the following year you are a bit more generous and not only fill but also need a bulge class so you can honour the offers accepted. This was unpredicted. Given tight space, timetabling becomes more difficult and some classes have to be taught in less than ideal rooms. So the next year you are a bit more cautious again to avoid a bulge.....but guess what, you more than fill, because that particular year you were first choice for a much bigger percentage, although that was impossible to know given the applications were the same. Given the building restrictions and space, that bulge class starts to create even bigger difficulties. So the next year, sensibly you only offer very slightly more places than you actually have. You've looked at the numbers carefully and can't believe there will be another bulge with the careful offering.....but guess what....it happens again. Very serious problems fitting them in now. So the following year, exactly the right number of offers are made - there really cannot be a bulge this year as it simply won't be manageable.....and guess what, this year loads decline their offers and go to the other schools they applied to, although the same numbers applied. And the school has to go to its WL.
I read an article in The Spectator today from a couple of years ago, which talked about the difficulty of WL for schools. Parents don't really see the issue - for them, WL is just another chance for a possible place at perhaps 2 or 3 or 4 schools they didn't get offers from - one might be their top choice, but it could be that none were. They don't really think about actually using a WL to fill a school or what's involved and the implications. As The Spectator said, schools can find a sizeable WL delivers very few people who would actually say yes to a place, because WL get used after the deadline for offers - this is the key thing. So some of those phoned never wanted the school which is now offering a place anyway, and the next few would have been interested but have already accepted another offer which they've totally come to love now, so by the time someone is found to take a place, they are actually significantly lower in ability than the last person offered to originally. And if you have to take quite a few of them, well exam result in that year group will drop. And worse can happen, a school can't fill from WL but has to start ringing candidates who never even got interviewed. There are threads here at the moment about top schools doing this last year - of course they keep it very quiet, becaue it doesn't fit with the image they want to project. But it happens.
So if you were that school which had loads of applications but 5 years of a roller coaster of bulge classes and not filling, might you not just look for a way to regain a bit of control - probably reluctantly because you know it won't be popular, but because it is better for the school.
If CIty hadn't used exploding offers this year and last, perhaps they would be facing more bulge classes, despite very careful offering.....and it's just not possible to accommodate them. Don't they have a duty to existing pupils to make sure they don't end up with too many pupils that have a negative effect on others already in the very space limited and restricted school?
There is more to this than parents having 3 weeks to choose between schools.