Sounds like a fun one for the shoot house. Bring a bin of commonly available items found in a classroom. The items can be randomly scattered around the room. 21 on 1 seems fair if only the aggressor has a firearm and the aggressor gets to choose the time to attack while the 21 perform meaningless tasks over a half hour period.

My guess is the most successful option would depend on how quickly the defenders recognize the threat and whether they gang rush the aggressor quickly enough to limit the damage.

I would think the video from the exercise would look like French police storming a super market with similar results.

Be safe.