Which school of magic do you prefer and why?

...I don't understand what the advantage of a wall of fire is - it's okay against random monsters, but it won't do much against serious monsters or a large number of them, and it's useless against a human player...

The wall is good when your unit is facing 6-7 enemy units (but not archers or those resistant to fire). And when you reach the enemy hero, you need to use other magic - there simply won't be time for the wall :D