Whatever triggered this thread, it is linguistically interesting. Generally, a grouping of fish is considered a 'school'. When koi are in a group, they do not exhibit many of the schooling behaviors of typical schooling fish. Startle them, or disrupt their environment, and they do school. But on the average day pondside, it is a bit of a stretch to say the koi are a school. They are simply a group of fish constrained by the pond dimensions to be together.
The behavior of the loose assemblage of a group of koi in a pond is more like that of a cattle herd than a school of minnows (or tuna). (They even graze!
) And when hungry koi race up for food, it does give an impression of a stampede. "Herd" can seem a more comfortable description of koi than "school".
The flexibility of the English language is that a word can be given new meaning by usage. Not all languages have that flexibility. There has to be a social acceptance of creative word use for it to occur. In English, there is cultural encouragement of creative uses.