Are there any spells I seem to be overlooking that would greatly affect the rankings? If I should be casting something that I am not, I would be interested to know what.
Slow, from Water I, is a curse which provides a benefit similar in strength to the benefit granted by the Haste blessing from Air I, and it costs about the same amount. The only problem it has is that it has to overcome spell resistance whereas Haste will always stick, though it isn't really a problem to overcome spell resistance if you're either high level or have one or two Prodigy traits.
Kill, at Death V, is perhaps the strongest single-target damage spell in the game, though it's a bit mana-intensive, slow to cast, and hard to get (and yes, I know about Mana Blast, but Mana Blast isn't guaranteed to be available in any given game, requires lots of mana to be a guaranteed kill, and costs about as much to cast; besides which, damage beyond the necessary amount to kill the target isn't really worthwhile). Great for taking out one or two powerful units (like Elemental Lords in Wildlands) if your supporting army can deal with everything else while you cast, not that great for dealing with armies.
Pit of Madness, from Death IV, gives you +1 research per essence in the enchanted city and applies the Fear curse to units participating in attacks on the enchanted city. Sadly, it costs you 1 growth, but it's an excellent enchantment for high-essence Conclaves regardless.
Sacrifice, from Death V, allows you to convert half the population of any high-level city you have into mana, at 1 mana per sacrificed citizen. Ever wanted to be able to use the expensive repeatable unit blessings from the other magic schools repeatedly? Ever wanted to make frequent use of spells with excessive mana costs just because? Ever wanted your Mana Blast to deal 10,000 damage per cast? This is one way of getting there, and as a bonus it allows you to make use of a resource (population) which is otherwise useless to you after certain breakpoints (unless you found Blood Curse).
If you're going for a melee Sovereign (or building up a champion with Earth Magic as a melee type), Diamondskin from Earth IV and Giant Form from Earth V are invaluable for allowing you to continue sitting on the front lines even if you don't invest in the defensive traits or are unlucky with weapon and armor finds (or both, to an extent). Earth isn't really a good primary school for a primary spellcaster, however.
Set in Stone, a city enchantment from Earth III, can significantly increase the production capacity of your cities if you're willing to sacrifice the resources and have an essence slot to spare. It's good for a low-essence Fortress pumping out fodder units, or for a very high essence Fortress pumping out elite units. I wouldn't use it on a mid-essence fortress pumping out elite units if I had a training enhancement I could put in the essence slot instead, though I might do it for a mid-essence Fortress pumping out fodder units depending on just what training enhancement enchantments I had available. It also isn't a bad enchantment to consider for hastening the building phase even on research cities, since if you have two or more base materials yield the enchantment is at least as good as Enchanted Hammers.
Regardless, which are best tends to depend on what you want to do:
Create elite units out of Fortresses? Air and Fire, then Earth, then Life; last come Water and Death since they don't do anything for this. (Why is Earth before Life? Because Earth has several spells which provide bonuses to the training of elite units - Aura of Might, Enchanted Hammers, and Set in Stone - while Life only has one, and I'd also rate the Life bonus as the weakest; Air and Fire are ahead of Earth because, even though they only provide one city enchantment each towards making elite units, that city enchantment is really good).
Magic damage? Fire, Water, Death, Air and Earth, Life. Fire before Water because you get to the decent damage spells with less investment in traits, and Fireball and Flamedart are both instant-cast once you have Savant while Blizzard still has a 1-turn casting delay; Water before Death because Blizzard is an army-killer; Death before Air and Earth because of Kill, Drain Life, and Touch of Entropy. Air and Earth are tied in my view more because I'm not really impressed with either school's set of damage spells; Life is last for the rather obvious reason of it not having any damage spells. Additionally, Fire and Water have most of the good strategic damage spells.
Tactical support magic? Air and Water, Life and Death, Earth, Fire.
Strategic support magic? Air and Water and Earth and Death, Life, Fire. Water and Earth have the strategic army stuns; Air has strategic teleportation, army disruption, and emergency army recall; Death has strategic Wither, which as far as I know is not resistible. Air, Earth, and Water additionally have useful unit enchantments, as does Life. Fire doesn't offer much here, unless you consider dropping Pillars of Fire, Starfall, etc on enemy armies until they no longer exist or are nearly dead to be "strategic support".
Summoning magic? Death, then everything else. After all, Death is the only school that still has any summoning spells (Touch of Entropy, which is probably more appropriately called a damage spell, but if it kills something it summons a Lurk).
For me, the ranking goes something like this:
Why is Life last? Because I usually play Empires, and so I don't usually have a good way to get Life Magic. The rest of it is based on me liking to use my champions as tactical damage and tactical support spellcasters, with damage being slightly favored. The top three ranks are nearly tied, while the bottom two are much less used.