The ability Schooling will not take effect if Wishiwashi is below level 20. It also must be above 25% HP or less. Healing Wishiwashi will also revert it back to its Solo Form. So Schooling cannot be disabled, but it can be avoided. Also note that the Ability-suppressing move Gastro Acid and the Abilities Mold Breaker and Neutralizing Gas will not affect Schooling.
Hope I helped!
Source (Mold Breaker) Source (Gastro Acid) Source (Neutralizing Gas)