I do not think Powder Blues actually school together. They are very territorial and the meanest of Tangs.
You can school Yellows, Purples, and even Naso and generally to have them you need more than 2. 3-4 is minimum
Blue Tangs and Powder Blues are not kept in schools.
I have kept up to 5 Tangs in my 150G. Even large ones... Naso, Blue, Powder Blue, Sailfin, Purple. The Purple and Yellow always fight, but The powder Blue dominates!