IMO the weaker schools are:
Divination - Trumped by numerous spells and heavy metal. Useful as a DM hint system, otherwise only as powerful as the DM allows it.
Enchantent - Oozes, undead, and constructs (generally) defeat the entire school, plus Mindblank stops the school.
Necromancy - Death ward trumps entire school. Some creatures (undead, constructs) make the usefulness vary.
Illusion - Trumped by Trueseeing (this spell is too easy to cast in 3.x) but can be useful if the foe doesn't have it and the illusion is used tactically. Great at low levels and when used creatively, but if the deception is revealed, you have wasted your actions. Shadow spells are neat, but are poor choices as a primary weapon.
This is not to say the other 4 schools are very powerful, but that they can generally be applied in more situations without worry of being rendered useless. Ideally, a wizard should prepare a few spells from each school, and his or her attack spells should vary in the saving throws they allow. IE transmutation spells almost always carry a fortitude save, but it may be wise for a transmuter to pack a few Suggestion spells just in case he or she fights Giants or other beefy creatures.
Granted, I am a fan of Evocation; When it comes down to it, everything has hitpoints. Everything. Monsters, buildings, traps, you name it, it has hitpoints. Take those hitpoints away - Problem solved. Not the safest bet for solving problems, but who doesn't enjoy a little collateral damage?
