Missed this 
My information is now over a year out as I am no longer involved with the parent council (as ds is now away at Uni), but the bad news is that the school is pretty much at capacity because it is has had so many placing requests.
The good news is that last year (ie 2018 entry) they ended up being able to take all the placing requests in, but I have no idea what happened this year, let alone next year.
I do know that someone planning on moving into the area over the summer was told that an S1 place for the August 2019 entry could only be guaranteed if they found a place to live in catchment.
Sibling requests will have priority - and with so many placing requests over the last few years, there will be lots of siblings.
Blame me for having raved about how good the school is 
It used to be that Glasgow would accept pupils from catchment primaries but they did a consultation last year to move fully to catchment areas rather than catchment primaries (determinate versus non-determinate - but don't ask me which way round that means
) and I think the conclusion was that geographical catchment is now absolute (ie catchment primary doesn't help if you live out of catchment). But sometimes they delay implementation of such consultations to reduce the impact on those that would already have been assuming where their child would go.
The only thing I would add is that there are a few primaries (well, at least one: Battlefield) which are dual catchment.
Best thing to do would be to talk to the school direct. They will be straight with you. 