Salut, xrachelbarber!
I'm britishpatriot, and I speak Italian fluently (semi-native) and I learnt French at school, taking my GCSEs early in year 9, and now, in year 10, I'm taking AS French. I've done a bit of Spanish on the side during the last year or so - nothing academic, just a bit of on-the-side work - which I'm hoping will look fantastic on my uni application if I can learn Spanish by then [w/o qualifications]
So I'm not the best-placed person to compare GCSEs and A/AS level standards between the languages, and my advice comes based on what I've seen of Spanish and what I know of French, compared to Italian.
Basically, I think Spanish and Italian are two very closely-related languages when it comes down to it, but to be frankly honest, when studying Spanish after having finished French, I felt as if Italian vocabulary looks perhaps more towards French vocabulary, but the grammar itself looks closer towards Spanish, and I feel as if Italian is a little less consistent when it comes to grammar, but that might just be because I have a more in-depth knowledge of it.
I would suggest that you consider taking GCSE Italian before you take AS Italian, and you should be able to do this relatively easily just by taking a couple of sessions in Italian outside of school and taking an exam at an examination centre, whilst probably reading a GCSE textbook in Italian (warning: I looked briefly for one last summer and couldn't find one, but I didn't look lots for it).
Whilst, of course, GCSEs aren't as important as A-levels, they show some understanding and knowledge, and are qualifications nonetheless. I would discourage you from jumping straight to AS Italian, just because, as you can probably presume, the vocabulary won't be there, and you'll get few marks for writing and speaking in a language guesstimated from good Spanish and French
Might it also be worth considering whether the new school might turn down your application to do Italian if you haven't taken a qualification in it yet?
Best of luck, anyway!
britishpatriot.