Unfortunately, campaign is very melee based. You can get reasonable xp as SCH using your AOEs effectively (I personally AOE Protect, Shell, Cures, and sometimes Gravity), although keep in mind that buffs and debuffs have to take effect in order for you to get credit. Pulling out your staff in the midst of casting will help you get a bit more xp as well, but the realism, however, is that any melee job will still get more xp than you.
Disclaimer: I'm not a SCH (although I plan to level it after BLU).
Purely on paper, SCH actually has superior Campaign potential than most melee jobs (DRG/mage being the exception), due to it getting pretty much everything that makes RDM top-tier in Campaign (Phalanx and native Enhancing Skill, debuffs, excellent MP-generation/efficiency), in addition to several things that RDM doesn't get (native Reraise, AoE-ability of spells). Try a build like this on the Monk Orcs in East Ronfaure[S].
NINs and THFs make good opponents as well; Phalanx decimates
the damage output of 1-handers, dual-wielders, and hand-to-hand users.
I'd start with just one Orc, but once you get the hang of it, you should be able to tank about 3-4 Orcs without too much trouble; and probably more if you use Defense Food.
Keep up Phalanx + Blaze Spikes + Aquaveil + Enspell, eat Fish Mithkabobs if needed, ride Aspir/Drain recasts (if tanking 4+ Orcs, Aspirga would probably give sick MP recovery). I wouldn't bother with Stoneskin at all (taking damage is the key to getting good Campaign XP on a non-DD job; or more accurately--in truth--in general): instead, control your damage intake rate by means of Slow/Paralyze. You can always Sleepga/Bindga/Graviga if you need to run for it.
Of course, this method won't really work until you hit 66 so you can have access to Phalanx/can survive against a Campaign mob solo.