One of the major desires of 8th ed was to tone down magic without rendering it irreleveant. As such GW rewrote the magic lores to remove the broken move unit dynamic. So how do these new schools of magic rank in power order.
8 Beasts: This school has only one good spell and one missile spell. The Signature spell is good but the ability is not releveant. Transformation of Kadon is the worst #6 spell in the without being remains in play which allows the opponent to dispell it at will.
7 Metal: The signature spell and the final spell are the only good spells in the school and its ability mades the signature spell a bad magic missile. Sure +1 to hit for you or -1 to hit for the enemy or +2 armor save are all okay but nothing worthy of getting excited about. The final transformation does kill a third of a unit so it pulls this school out of the worst spot.
6 Fire: 3 missile type spells, and one good spell. The ability of the school is hard to use but not awful. Too bad the final spell sucks or this would have been a good school but a scattering tempate is a bit lame.
5 Heavens: If you want to blow things off hte table this is your school, with 3 good damage spells (2 at S6) the ability to kill fliers, and the great wind blast spell this is a good school of magic. Unfortunately is only the best of the lower tier of spells.
4 Light: A good signature spell, a useful ability, and the best buffs in the game. If you are planning on fighting this school will make you win these combats and it has enough missiles and the most intriguing spell in the game, net of Amyntok.
3 Death: This school majors in character assassination with 3 spells to kill enemy characters and a huge template spell which could kill a whole army. Further it still has doom and darkness which is an essencial tool in the demon bomb armies.
2 Shadow: The ability is not great but the signature spell is better than most people would think. The pit of shades now kills even more models and the rest of school is good for winning combats. There is not a bad spell in this school except for steed of shadow which is still good.
1 Life: It is not even close as this school has the best final spell, the best ability and a spell to ignore miscasts. A good missile spell and +2 toughness are gravy while Regrowth is raise dead for the rest of us. Dwellers below puts this over the top as by using strength rather than iniative to kill whole units this spell can be devistating especially to opposing wizards.