Schooling in Pokémon Sword & Shield

When it has a lot of HP, the Pokémon forms a powerful school. It stops schooling when its HP is low.

Wishiwashi at level 20 or higher uses School Form while above 25% HP.

A Wishiwashi at level 20 or higher changes to School Form when entering battle or at the end of a turn if its HP is above 25% of its maximum. At the end of a turn, HP at or below 25% makes it return to Solo Form. It is not a start-of-turn check.