Hi, I live right on the edge of the catchment area of an outstanding school. For the past 6 years I've been in and out every other year. I'm sure that this is mostly down to siblings. I wonder if anyone would have good advice about how to find out how many siblings for 2018. Thanks
Your only route would be to ask the school or admissions authority how many pupils were admitted to the school under this criteria for the last few years, and if they have an idea how many there might be next year - the admission authority should be able to tell you exactly how many there have been under specific criteria historically. They might be able to make a guess for next year, but they won't know exactly. And 2018 would really be a finger in the air!
You would also need to know if there are any bulge classes in the years above, as they will swell the % of intake that is siblings after a couple of years (say when the bulge is in Y1/2/3, that will swell the % of siblings in reception if the bulge is a one-off).
Thing is, it's not just siblings that affect things. It's also down to all of the other admissions criteria - usually "looked after" children/special needs naming that school, then sibs, then distance. Even if you know the number of siblings you don't know whether three of those families may move away from the area, or put their kids in prep schools, for example. You don't know how many adopted kids or children in care, or with specific additional needs there are and, crucially, you still don't know how many children live just a fraction closer to the school than you do.
Sibling numbers do have a big impact (assuming sibling priority is part of the admissions criteria) but it's not the only factor and the only way you're going to know whether you will get a place is to wait until the day they release admissions results. That really is all there is to it.
By all means you can try asking the school - some will tell you what they know, others won't - but it won't really make a difference. If it is the school that you really want your child to go to, then put it at number one on the application form. Whichever school you apply for, I'd avoid giving your child an impression that they will definitely be going there until after you have a place allocated. So honestly, it really makes very little difference if you know.
(Also, make sure you actually visit the school. I really hated our local "outstanding" schools and they would not have been right for my child.)