My understanding is that keeping a large school can theoretically work, but - as mentioned above - there are a number of things to keep in mind with schooling:
- The number of schooling fish in the tank (I've heard odd numbers are preferred, and the preferred number of fish that I've seen seems to be 9 to 11 at a minimum; the more fish, the more diluted the aggression is between them)
- The size of the tank (for quality of life purposes for your fish, bigger is better - yes, some animals will be calm when shoved into tiny spaces with large numbers of conspecifics where if there was just one or two conspecifics, it would be a deathmatch, but they obviously wouldn't be happy in that situation long-term)
- The amount fed, the quality of the feed, and the frequency of feedings (basically more food = less aggression; and better food = better health = happier, less aggressive fish [theoretically])
- Tankmates (big, scary tankmates that the schooling fish could view as a threat may act as an outside force that keeps the schooling fish focused on not getting killed rather than on fighting amongst themselves)
- The scape of the tank (lots of fish need lots of places to hide/sleep - the more hiding places, the safer the fish feel; line-of-sight-breaks can also help with feeling safe)
That's all I've got for the moment (and pretty much all of it has been mentioned above), but basically - to my understanding - it's a balance of making the fish feel threatened enough by external sources to prevent infighting while also making them feel safe enough (largely through their numbers, the tank's scape, and food security) to not be too stressed. If you're able to strike that balance, you should be able to see schooling behavior (to the best of my current understanding).
