Wishiwashi School Form is a transformation from Wishiwashi-Solo. It can only be activated in battle, due to Wishiwashi's unique Ability, Schooling. This Ability activates when Wishiwashi is above Level 20 and has over 25% health left. Wishiwashi's stats dramatically increase upon this transformation, and dramatically decrease upon reverting back.
It's a powerful Pokemon, but one that can be ridiculously weak. That's how Game Freak balances Pokemon like Wishiwashi -- by giving them some sort of hindrance.
Wishiwashi is not categorized into any specific category, unless you count species names, but that's getting technical. It can be placed in numerous categories, but it has never officially been given a category like "pseudo-legendary", or the like.
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