Reddicediaries (that is a tough one to rewrite
) is right.
To add more juice into his answer, Paladin will be able to cast everything, which he will be able to put into his spellbook. He will cast Dark Spells, Fire spells etc. Only spell that are out of his reach are those, that are restricted to some other mage. For Example [mwcard=MW1Q05]Demonhide Armor[/mwcard] is for Warlock only, thus Paladin cannot put it into his spellbook thus cast it in a game
Now, for his schools. Holy 3 War 2 and Trained in all Holy creatures.
That means that he will be able to put into his book [mwcard=MW1J23]Temple of Asyra[/mwcard] or [mwcard=MXSTX2FFJ02]Altar of Carnage[/mwcard] while paying exact prize in Spellpoints. But for spells like [mwcard=MW1I22]Resurrection[/mwcard] or [mwcard=MWSTX2FFC05]Anvil Throne Crossbowman[/mwcard] he will pay Double the prize in spellpoints. The last thing "All Holy Creatures" means that even mighty Angels such as [mwcard=MW1C39]Valshalla, Lightning Angel[/mwcard] or [mwcard=FWC13]Selesius, the East Wind[/mwcard] will be for exact number of spellpoints. So 5 spellpoints in this matter.
One last thing, which is not actual yet is, that if there will be some creatures with mixed schools, in same manner as [mwcard=MWSTX1CKC01]Bridge Troll[/mwcard], for example Holy and Fire, you will be able to put it into your book for standard number of spellpoints. Lets sey if the wording was "All Nature Creatures", he would be able to put this troll for 3 spellpoints. It is because he is Nature AND also a War at the same time. Reason why it is not actual yet is, that there is not a creature with mixed schools like Holy and Fire etc.
I hope i didnt add into confusion with my last explanation