The way you put it having an all-girls school sounds pretty awesome? Like I'm used to always having boys around in lessons but they're so irritating sometimes omg, but then it'd be so weird if they weren't there haha
I love black dresses for prom personally, not so many people go for it so it stands out more.
A girl at my school went in a full lace black dress to prom this year and it looked so amazing omg
I'm not sure about the colour yet, like dark green and burgundy are the best colours for me, but some styles only really suit paler colours.
As for style, I'm tempted towards a long dress, but no idea what particular fit yet, I like mermaid silhouette ones but they can be way OTT with dumb netting and stuff, so maybe just a chiffon a-line one. It's a struggle haha, this is probably going to stress me out more than exams?