1) Earth. It's great. What makes it great though is not oomph power of spells, but versatility. It features great healing spells, great debuffs (poison spells sucks out Evade, Acid Splash - Armor), removes one of the most annoying illments in the game (poison) and has reasonable damage spells. Most of them work best at the beginning and midgame however, although Acid Splash passes through even most resistant enemies, making it a skill of choice for your "1" key.
2) Fire. Either Expert (Burning Determination) or GM, in my opinion it is the school of choice for DPS spellcaster, including one with GM Focus. You well spend a lot of time in the game killing undead, various elementals, orcs, nagas and human-type enemies who are surprisingly resistant to Air. Sucks to be a GM Air-Freemage when a pitiful blackguard resists 3/4 damage your GM AoE does. With Fire, it seems a lot easier to burn through everything. I mean, just how many dwarf-type enemies you've met with innate fire resistance? How many Fire Elementals? Like, ten, counting the boss and some at the mountains? Shadow-type also seems vulnerable to Fire (naturally).
I would probably combine GM Fire with M Light, since usually you'd use Light when your Earth support can't hold, and if you meet Fire-resistant enemies, you can just spam Celestial Shield.
3) By the way, Light. Celestial Shield is a spell of choice for many, but there is also one little spell called Radiant Weapon, an Expert DPS spell. It's damage is weak (~100 for GM if you hit Light-vulnerable creature), it's not cheap (20 mana), but what is that... "does not allow enemy to block this turn"? Yeah, the turn you casted it, enemy can't block. And since it's Light spell, it almost never gets resisted. So what I did was I cast this spell and then attacked target with my Blademaster. No Warfare skill needed. Add an Acid Splash and you can reliably mow through anything with little mana&effort.
4) Dark. Oh Dark, woe has done they to yeth... where is my Dragon's Breath? GM spell does 22 damage per turn, for 2 turns? Is that a typo, shouldn't it be 220? And 20% damage reduction does not seem that much when enemies damage is double or trible time your health...
A properly build party should outdamage various regenerations, imo, so Purge is only a no-brainer for Crystal Spider fight, since it's buff is so obviously overpowered. Casting Agony seems like a poor man's replacement for a decent cast of good DPS spell, and let's face it, your DPS monsters would probably kill that mob anyway if you devuff evade/block/armor away. Sleep is semi-useful in midgame when you are outleveled, but if you go where game's plot wants you to go, you probably could work around it, plus there are other various abilities to CC enemies (from Primordial, Earth&Water). And there is detection of secrets... which is useful, but you don't need to pump school too much for it, and you get one for free once you finish Elemental Forge.
So, Dark. I GM'ed it out of old-school feelings, better I did't.
5) Air. The DPS choice for Freemage, since he can't GM anything else of worth. It is almost purely made of various attacking spells which, if not get resisted, hit pretty hard. Shadow-type enemies which you will fight a lot would get blasted by it, so a Freemage with GM Air, built to last, can basically waltz through Tomb of Terrors & whole endgame laughing maniacally throwing both AoE spells (Cyclone&Storm) and Chain Lightning which does obscene crits if enemies are grouped together.
When Air gets resisted you are a very sad Freemage though, and it probably will get resisted quite a lot during game (I'd say 30-40% of monsters seem to enjoy good Air resistance, but maybe it depends on a time you actually start using Air extensively, which is when you reach Crag/GM it).
A hint about Eagle Eye. Since ranged weapons seem to suck big, usually characters will have easy time to open Might&Magic secrets, but not Perception passages. For that, exists this spell, which you can cast on your DPS guy with high PER to open a passage with 100% of sucess.
6) Water. A fairly reasonable damage school with de-buffs, but with notorious reputation for poor design in it's learning. Good thing characters who can GM it can also GM Earth/Air and dabble into Light/Primal, meaning you don't have to be greedy on skill points.
It's damage is good, particularly from Tsunami. There aren't many creatures who are completely resistant to Water, except obvious water elementals/nagas.
The problem with Water though is it doesn't work well with other DPS schools. Fire likes your enemies clustered in a row or around you, and Air loves when they are near you under various ticking AoE's. So, that's that. Water, you are fine, but I could do without you - a focused Earth/Light support char is enough to overheal even when being surrounded without Liquid Membrane, and Water Flows Freely can be replaced with Burning Determination/Items.
7) Primordial. This school like Earth focuses on versatility, but instead of healing and debuffs it's about buffs, CC and utility. Generally one can do without it, but having it at least on Master makes life a lot easier since you can teleport your way around to shops and taverns, repleneshing supplies and jumping back to quest givers.
A GM at Primordial will always have something to do around, either popping enemies with Implosion/Mana Surge (which DOES get resisted a lot, but is still better than doing nothing), keeping Hour of Power/Heroic Destiny running for your melee's, CC'ing stuff and so on.
For my Druid I GM'ed Earth&Water, but I noted Druid can also GM Primordial. Since it's so easy to get promotional quest for Druid I am leaning towards GM Earth/Primordial druid now. Primordial spells can, or should like Beacon/Identify, casted outside of combat, so having Druid cast them and then just keep Regen flowing makes for an ultimate support.
And of course it means Hour of Power at GM level as soon as Act II.
As for other skills, they are more for tweaking characters. For example, Shaman can probably get away without Mysticism at all. And Focus is probably only good if you go full DPS. There are sure more winning combinations in game than "buff Blademaster, kill boss in 2 hits" I used, I wish game was more challenging so you'd want to use them and positioning skills more.