Considering Art A-Level without GCSE - Portfolio help!

Hi, so I'm currently a homeschooled student taking (mainly) iGCSEs, and I'm expecting to join an actual school for Sixth Form. I've also been to some of their open events, and in one of them, I talked to some students about the Art A-Level. According to them, it actually seems more freeing than I'd expect - I mean, one of them took up 3D modelling as a medium!
Anyways, that encouraged me to consider Art as an A-Level, but as the title says, I'm not studying the GCSE for it.
I DID ask the school's Art teacher about it, and her response was to make a portfolio.
Honestly, I have no idea how to make one, especially since my medium is digital art (specifically... the dreaded anime/manga style). I did email the school for advice (they haven't really gotten back yet), and as the 3D modeller proves, they're really relaxed so I don't think they'll be restrictive on what I draw. But I feel really pressured because Art as a subject seems really advanced and again, I have no idea what to put in my portfolio, or how I should format it.
Any tips will be appreciated!

I initially applied for A-level Art when I didn't do GCSE Art. I ended up not going through with it but I did get accepted to do it. I mainly used watercolour, but also used coloured pencil, acrylic paint and pen. I printed out some photos and put them in a fairly cheap sketchbook from WHSmith. From looking at art courses at colleges, they were not a fan of anime/manga (explicitly stating they wouldn't consider it). I would put your best pieces of art and include anything that isn't in your usual art style if possible to show some variety in your artistic skills.

Hmm I honestly get the no anime rule, but I have heard of a few schools who did allow their students to pursue that style, so I'm just waiting for the school to give the green light. Whether they do or not, I'll try my best to experiment with mediums and other art styles, so thank you for your input! Do you mind if I ask for the amount of pieces you submitted for your portfolio?

It was work I built up over about a year because it was a hobby. It was probably around 10-15 pieces of art alongside some photos I took over the course of a couple of years.