All the pupils were told that they had to dress up as Royalty and if they couldn't manage that they at least had to wear a crown. Lots of people have gone out and bought specific fancy dress outfits of knights, royal guards and princesses. Personally I didn't see the need , it's not as if a guards outfit would really get worn again, well not in this house anyway.
So I bought a couple of masks and crowns off a website which just happened to have free delivery and used outfits we already have so the boys could go as a casual Prince William and a soldier Prince Harry. Personally I think for £4 they look rather spiffing don't you?