Rummynose tetras are pretty much universally recognized as the tightest schooling fish out there.
However, how big is the tank? Fish school as a defense mechanism against predators. They like to be close to each other, so that each member of the school can see all the other members. In that way, they have a shared vision that protects the entire school. Generally, in tanks shorter than 48", the fish can spread out across the tank and still see each other, and therefore won't exhibit their full schooling potential. Point being, if your tank is smaller than a 55g, you likely won't even see that great schooling you're looking for.
However, how big is the tank? Fish school as a defense mechanism against predators. They like to be close to each other, so that each member of the school can see all the other members. In that way, they have a shared vision that protects the entire school. Generally, in tanks shorter than 48", the fish can spread out across the tank and still see each other, and therefore won't exhibit their full schooling potential. Point being, if your tank is smaller than a 55g, you likely won't even see that great schooling you're looking for.