If you're looking for debuffs, this is the school of magic for you. Every spell line in this school includes numerous debuffs. The Slowing line of spells starts off with Weakness, which inflicts penalties to attack and defense on the target as well as slowing the target's movement. Things become nastier from there, as the Slowing line also includes a single-target and area effect paralyze spell. These spells are just as good at crowd control as the spells in the Telekinetic line of the Spirit school.
The Hex line can also be very useful. The standouts are Misdirection Hex and Death Hex. Misdirection Hex causes an opponent's normal attacks to automatically miss - only some special attacks and critical hits will make contact. Death Hex has the reverse effect. All normal hits to an enemy with Death Hex become critical hits. Both of these Hexes often stick to bosses, and they'll make your Warriors and Rogues on the front line extremely happy.
The Sleep Line consists of spells which, as you'd expect, cause the opponent to become disoriented, fall asleep, or have nightmares. It includes two crowd control spells, Sleep and Waking Nightmare, as well as two minor debuffs, Disorient and Horror. While the Sleep line is not weak, I feel that it pales in comparison to the Slowing line and to the Spirit school's Telekinetic line.
Finally we come to the Draining line, which includes spells which manipulate life. Its lowest level spell, the Drain Life spell, is a very hand spell for Mages. Set it to automatically cast at 25% health by using tactics and it acts as a sort of instant heal. The Draining line also includes Curse of Mortality, a nice debuff which makes it impossible for the target to be healed, and Death Cloud, which does spirit damage to all opponents in an area. While both of these are decent spells, the Primal school is better for dealing death, so I generally only recommend picking up Drain Life.