Hard to give an opinion without know more details.
If he had his starters & best players in the whole game (which I'd guess he probably did), yeah he's an ass and the firing is justified for display what is, IMO, the lowest of the low in poor sportsmanship.
OTOH if he was playing his bench by the time the lead got massive, and the other team was just that bad (which it sounds like they could possibly have been), well, I don't really know what could have been done. I'm don't know offhand of any rules that allow a game to be called early if one team has a massive lead, and even if there were I think if I was playing that it would be more embaressing to have them shut the game down and say "you just suck too much" than stay in and actually try, even if you take a beating like that.
It really sounds like the Dallas school needs to be in a different division or something, where they actually have a tiny chance to compete.