... It's in the name.
There are Eight schools: 5 Magic (Earth, Fire, Water, Air, Witchcraft), then there are the Warrior and Survivor (Ranged and Rogue), plus one "Special" school which is for non-categorized skills, such as those you acquire through the course of the story.
The 5 magic schools are boosted by Intelligence, the Warrior is boosted by Strength, the Survivor skills are boosted by Dexterity.
Skill type - Ability Required
Earth - Earth Elementalist
Fire - Fire Elementalist
Water - Water Elementalist
Air - Air Elementalist
Witchcraft - Witchcraft
Warrior - Way of the Warrior
Ranger/Rogue/Survivor - Way of the Ranger
Spears and Polearms, Bows, Daggers, Crossbows are all DEX based. Staves are INT based, everything else is STR based.