I believe that both Attack and Dexterity stats concern Melee and Ranged attacks rather then Magic attacks. Spells hitting should be influenced by your mastery in the appropriate school of magic, by your enemy's Magic Resistance, and also by your Wisdom (which increases spell penetration at the Journeyman mastery level). Spell penetration can further be raised by spells like Bless, Insight, and Archangel's Power, or by the Potion of Wizardry.