it's just an impossible system to navigate (6th form) as it's done on the UCAS system here and not on the 'normal' schools admission system.
That means the schools don't know how many children hold places elsewhere and how many will turn up on the day. So for example, there are 6th form colleges here which are quite good but have a lottery application system (ds didn't get a place at the one nearest us, it's just a random pick out the hat system). But that doesn't mean he gets a higher priority for the ones where you are prioritised on distance. So someone who got in to the college can maintain a place with the nearest school and only on the 23rd August decide which one they are going to take. Ds therefore has to go to the school at a certain time, see if there are any places left, see if he has the grades and more importantly, see if there are spaces left on his course.
So he could get to the school he wants to go to, find he has a place based on distance but then be told that Physics is full so he can have a place but not do the subjects he wants.
it's actually giving me sleepless nights
when we did it with dd, she got the last place on English A level and I have no idea what we would have done if they had told us English was full and she couldn't do it!