sixth form applications

hi!
i just had a few questions about the sixth form application process as I'm a bit stuck.
say i have 3 options for sixth forms, sixth form A, B and C.
sixth form A is the hardest to get into and I don't have enough points so I apply to sixth form B instead.
On GCSE results day I find out I have enough points to go to sixth form A, can I turn up to the school on the day and apply?


Also what if I tell each of the sixth forms that they are my top choice so I don't lose my offer, eg sixth form A offers me a spot so I turn down sixth form B's offer but on results day I find out I don't have enough points for Sixth form A so I end up applying to sixth form B, would they still take me?


sorry if it's a bit complicated, i'm very confused and not sure where to find this info