With the A level reform, AS levels aren't being offered "under normal circumstances" at my sixth form. I really want to take them, as I want them as a solid indicator for my academic ability when it comes to applying to university. I want to apply to Cambridge, as I am predicted 10A*s at GCSE. Also, Cambridge offers the course that I really want to do (Natural Sciences). I know that you won't really be disadvantaged without AS levels (as some schools do not offer them now), but I really want to take them anyway (I am 100% sure I can put the effort and hard work into it, so I can achieve A grades). The point I'm trying to make is that I know fully of the implications of taking AS with the A level reform.
So my question is, what could I say that would convince them to let me take AS levels? Is there any real reason why they wouldn't let me, besides them not believing that I can achieve 'good' grades? If I go to them with, say, 10 A*s at GCSE, and say that I really want to take AS, do you think they would allow me? What reasons would they have against that, really?
Edit: In case it was ambiguous, I am in year 11, starting year 12 in September.