The definition of "school" and "shoal" is not always clear. The ones I tend to use apply "school" solely to marine fish, and this means the species remains together 24/7, they hunt/feed as a pack, etc. As far as I know, no freshwater fish does this, at least not strictly. "Shoal" on the other hand refers to the freshwater species habit of remaining in large groups that may form a "pack" for various reasons, but also separate partially and/or completely, and they never hunt as a pack with the sole aim of bringing down prey together.
I acknowledge there are sources who reverse these definitions, or who do not apply them like this at all. This is why I tend to use shoaling for those freshwater fish that must have a group to avoid stress in general, such as the characins, cyprinids, some catfish, etc. It is certainly true that the larger the space they are in, the more they generally remain close. And when threatened they will tighten the shoal even more.
Many of these species do not actively swim much to begin with. Rummys as I mentioned are the best example of an active-swimming fish that does tend to remain in a close group while swimming through the tank. Many others, such as cardinals, the Rosy clade species, and so on are quite sedate, remaining in close proximity under the shelter of plants, branches, etc. Rasbora are like this too.
Byron.