How is this going to work?

Hello all,

I am currently a Year 10 student at a local comp. school. My school is currently telling my year that we need to start thinking about our post-16 studies. I, however, am thinking of switching schools to my local grammar. My question is: when do I actually have to apply? I really don't know to much about the process and some advice would be really great.
I also have another problem that I would love to get some advice about. One of the main reasons for me switching schools is that my current school can no longer facilitate my needs. Basically, it's rare in my school to have more 10 A*s at GCSE by 16 and at the same age, I will already have 2 A-Levels (hoping for A*s in both) in Maths and Further Maths and possibly also a Certificate of Higher Mathematical Sciences from the OU counting for 1/3 of an Honour's degree. Would I have to explain all this to my potentially new school? I am worried that even the grammar might reject me and I don't know what I would do if the best school in my local area rejects me. Any advice for the future would be really appreciated.

Thanks