How do sixth form applications work

Do sixth forms look at your predicted grades or your mock results on your application? I don’t really get how it works…. If you get rejected, but your actual GCSE results exceed your mock/predicted can they still accept you? I’m predicted 7s and 8s and I want to apply for a grammar sixth form, but I think my mocks results won’t be what I want, so if they reject me, but I get higher grades in the real GCSEs can I still get accepted? Thanks