I am often asked which school of magic is best against such and what-not army. Well I have a few simple rules about magic - move spells win, and missile spells have better be no armor save (otherwise the mage's point cost would better be spent on crossbowmen). As such the best schools seldom change and opponent's armies do not affect this much. So here they are in my opinion with some notes:
1 Necromancy -- multicast move for 7 is ridiculously strong and the rest of the school is none to shabby either.
2 Tomb King -- always cast spells with a move and a fight spell.
3 Shadow-- 2 move spells, a no save missile and pit
4 Big Waagh -- everybody moves, warpath and a spell to pick out an individual model not bad
5 Wood Elf -- Move terrain is good but the call of the unt is universally good
6 Beasts -- particularly if both armies have cavalry or monsters otherwise do not touch it
7 High elf -- Particularly against a magic heavy opponent , Flames gets all the attention but Vaul's unmaking is a game winner
8 Little Waagh -- a move unit spell and a few decent situational spells
9 Metal -- Situational as it is good against knights, chariots and warmachines and even has a turn off magic item spell
9 Light -- Situational great against deamons and undead, also has a lto of good occasional use spells
9 Heavens -- Situational a great addition to shooting armies or fast armies with lots of attacks
10 Death -- Short range but lots of interesting missile type spells and lots of things combe with doom and darkness
11 Skaven -- the warlock engineer gets the good spell but this is really only a missile school.
12 Tzeentch -- a bunch of mediocre missiles
13 Nurgle -- interesting spells but require a failed attribute test to make them work
14 Life -- terrain can make this better and it is good against wood elves but not great
15 Slaneesh -- weak in most ways
16 Fire -- when my opponent takes fire I just know I am going to win