Thanks, I've made some progress based on this. It looks like there are actually
three versions. V2 from 2005, the 1999 V1 for PC and Mac, and the real original V1 from 1992 for Acorn Archimedes.
I was only seeing screenshots and videos from this version, which looked too fancy:
https://www.youtube.com/watch?v=gJXLfXZuBiA But I would have played the Acorn Archimedes version, which looks like THIS:
https://www.youtube.com/watch?v=qMi0QoLB4gM I totally remember playing that game at school, which is a good find in itself (although I wish there was footage somewhere of the full version),
but I don't think it's the specific Aztec game I'm looking for. However, the same developers - Sherston Software - made another game for Acorn PCs just a year later
called Aztecs. There's almost no information on it that I can find, but here's a copy on eBay:
https://www.ebay.com/itm/Sherston-Software-Aztecs-for-Acorn-RISC-OS-computers-/184055274196 Considering the others asking are both from the UK where Acorn PCs in schools were common, I bet that's the game we're all looking for.
I've attached the images from the eBay listing here in case they go away in the future. Sadly I can't find so much as a screenshot anywhere.