Wishiwashi's base form has a base stat total of 175, absolutely horrible...
School form however it's base stat total jump to a whopping 620... all of those stats go to his Atk now 140, Sp Atk now 140, def 130, Sp Def 135.
For comparison Wishiwashi's school form has a higher base stat total than Mega Gardevior/Gallade Mega Aerodactyl, Mega Ampharos, and Mega Steelix. And only 5 points shy of mega Lucario and Mega Venosoar.
TL:DR Wishiwashi school form is a monster...