You clearly had some kind of PC like a PS/2 or a PC clone. Something pretty new for 1990 if it was running EGA games like Number Munchers. Schools wouldn't yet be running Windows 3.0, but since DOS was seen as kid unfriendly they likely had installed some kind of a 16 color program frontend for DOS. There were many of them in the 80s and 90s, most not specifically for kids, and were just software that ran on top of DOS and let one use a simpler menu interface to run DOS software and maybe perform DOS file management tasks. The keyboards having built in trackballs would be a little weird/early for 1990, but there were some keyboards, generic Taiwanese keyboards I believe, which had built in trackballs in the early to mid 90s. Which might hint that you had totally generic, locally built computers or computers assembled from Taiwanese parts from a smaller company like DTK or Northgate.

I would start by looking over the MECC PC software library, especially if this game was polished and actually any fun.

Hmm.. my time in elementary spanned late 80s to late 90s and there were a couple of different machines during that time including 3.1 later on (I think WFW). I remember number munchers from almost Kindergarten but I was part of the "catholic separate school board" in Toronto Canada. Now... don't get my wrong, our economy is absolutely balls for a Capital of a 1st world country and the school budget was always a point of contention but I have trouble believing they sourced random crap and threw it together, especially if they had WFW later on. Our libraries had at least a dozen machines, probably closer to 15 and 1 in each classroom to boot. Is it possible it was Apple or something or perhaps my memory is just wrong? The resolution was definitely low and pixelated and I remember a lot of Cyan and lots if Icon based stuff on the interface (vague as hell 🤣).

I just checked the MECC games and I don't think it's in there. There are only a dozen or so and none have to do with dungeons or mazes.