How do grammar sixth form applications generally work?

I'm currently applying for sixth forms, specifically grammar schools, as i've always been a grade 7-9 student, however, my real mocks are next month and the applications close this month, so the only grades that could be sent for my reference were my mock-mocks which I did terribly in (grade 3 RS, Grade 4 maths, which is not at all normal for me) since my parents had just split and I wasn't in the right headframe.
I was wondering if it matters, if I can change it later, or if they send initial offers based on those grades? Basically, am I screwed?

Hi,
I tried applying to my grammar sixth form but my Nov mocks were really bad and I also saw that the deadline was January, before the second mocks. I didn't apply but day of results day I emailed the sixth form showing my results (which averaged 7.75 from the top 8 which I was told they look at + inc the obvious Maths/Eng/Science grades) and my what subjects I wanted to take.
Did not expect much, I think I got an email telling me about one of the subjects I wanted had to be changed which I obliged to and basically got put on a waiting list?
Literally, a few days before school started in September (I was planning to go to my secondary's sixth form by now), I got a call from the school stating a place opened up and I can come in to enroll on the weekend then start in a few days. I got like 45 minutes to make the decision to switch over because I'm guessing they had more people on the list they'd go to if I said no.
At the end, I said yes, went there to enroll with whatever docs they needed me to bring, emailed the sixth form I was originally supposed to go to about the decision and that's it.
Currently in year 13, don't regret it at all. I got a lot more opportunities at this grammar than the regular one I was going to go

That’s amazing, you got so lucky with that, and that makes me feel so much better. Perhaps after my next mocks I could contact each school to update them on my grades then, or if not, after results day I’ll try and get in contact again. I kid you not, I literally sighed a sigh of relief as I read this, so thank you