As far as I know, the school-specific skill bonus has no bearing on your ability to pierce resists. Spellpower is determined by caster level (overwhelmingly this - this is why bishops will always be outperformed by pure casters, every mechanic in wiz 8 checks level before anything else. every level difference between you and an enemy is essentially +10 to all their resists), the innate level of the spell, the power level you're casting at (which is mainly determined by realm skill, not school skill), and, believe it or not, the mana cost of the spell. Literally, spells that cost more mana pierce resists more strongly.
Basically, if you're going to use a bishop as your offensive caster, then make sure the rest of your party is all hybrids/advanced classes. If your bishop is noticeably behind the rest of the party in levels, as will be the case if you have them mixed with a bunch of fighters/rogues/pure mages, then your bishop will never be able to land CC spells as he will be 2-3 levels below all the monsters scaled to your party.