Gunnery school of nuln

Are the school of nuln an officiall warband?

Have a look at this thread for the "officialness" and "trustworthiness" of warbands and hired swords.
https://boringmordheimforum.forumieren.com/t816-reference-the-watchtower-mordheim-rules-index

The answer to your question though is "no". Although the Nemesis Crown supplement was comissioned by Andy Hall, head of Specialist Games, the contents never made it into the official list. Well, there was never another Rules Review (and there won't be anytime soon) anyway.

Check out the Mordheim Rules Review PDF on the GW website for a list of what is "official". At this time this really shouldn't be of any relevance to any game group though. There is no reason to dismiss certain rules which look like they might add a good portion of fun to your gaming experience only because GW abandoned everything and didn't put an "official" stamp on it.