I think how it works is that when they’re just chilling side by side they’re a shoal, and when the shoal starts moving it becomes a school. (Apparently the words are closely related, both being related to Old English scolu, meaning multitude.) They are a bit like a married couple, generally preferring to be close to each other but occasionally taking brief jaunts to different parts of the tank before coming back together. (But what’s this? Ah, it’s a leaf. What did you find? Some gravel? Nice.) When they do school, the bigger, more able-bodied fish usually leads the way. There’s a hydrodynamic benefit to schooling, as there’s an aerodynamic benefit to flying in a V, so I wonder, is the bigger fish lead fish because the smaller, weaker one wants to save energy? If so, is this a conscious choice the small fish makes—or that both fish make together—or did evolution just program all Carassius auratus (“gilded carp”) to let the biggest, healthiest fish swim at the head of the school? Do fish generally swap out who leads the school, as geese take turns flying at the head of the flock, so no one gets too tired?

Regardless of the answers, isn’t life just the best? It’s so interesting! It just brute-forces the problem until it comes up with something, and one of us ends up being a biped with lungs and one of us ends up being this shiny thing that breathes water, and those are both correct answers!