Although it is hard to comment on the individual spells without seeing the overall shape of the game, a few comments none the less:
Paragon – should there be a limit on how high a level can the champion get from this spell?
Steal Spirit – should this be a Death spell? It seems like a fairly evil thing to do.
Titan’s Breath (Air 3) – the effects seem rather small for a level 3 spell.
Cloud Walk (Air 3) – teleportation being present in only one school of magic seems rather powerful.
Storm (Air 4) – seems worse than Fireball (Fire 3) spell – it’s random, and does less damage. Is this intentional?
Tornado (Air 5) – it’s probably really irritating to have to recombine your army afterwards for the player. It also means that an AI who stockpiled a lot of mana would be really irritating to fight against, as they would just cast this every turn.
Blindness (Death 2) – Should this completely prevent ranged attacks, judging by the name of the spell?
Curse (Death 2) – Seems relatively weak compared to Wither.
Contagion (Death 3) – Seems like a very fast-scaling spell, and easy to abuse by the player; put one spellcaster on a horse, cast this spell, and run away from the enemy all the time. Considering that it is possible to go heavily on the Death shards, this can be very irritating.
Touch of Darkness (Death 3) – what does Intelligence actually help with? I don’t see any spells being influenced by it.
Tremor (Earth 2) – another very irritating spell to go against. Could an AI simply lock down the player with this?
Focus (Fire 3) – seems a little out of place for Fire, considering that overall theme of the school is to do damage. Also, could this affect allies, not just the caster?
Mantle of Fire (Fire 4) – should this be affected by the shards? Also, should this be both a strategic and a tactical spell?
Regeneration (Life 1) – scales really fast, making it meh with no Life shards, but rather powerful with two of them. It seems like it really needs to be a level 2-3 spell, considering it allows a fast unit to just run away and heal up during combat.
Growth (Life 3) – is this the same as Death’s Berserk? Can this be cast on allies only, or on enemies as well?
Shrink (Life 3) – can this be cast on enemies as well?
Freeze (Water 2) – this is almost complete immobilization, and as such, is quite powerful. Compare with other immobilization spells; perhaps this should be at level 3.
Grip of Winter (Water 5) – a powerful spell. If an AI casts it against the player, how can the player react? Can the player prevent this somehow (i.e. an overland enchantment spell that makes it difficult for other to cast Earthquake or this one against them)?
Dirge of Ceresa (Death 4) – considering the power of this spell, it seems as if the Death Adept faction becomes a one-trick pony – it is much more efficient for them to convert all their shards to death, and this spell becomes an “I win” button. This is better than Fireball, for example.
Blood Rage (Death 4) – Any particular reason for why this would be a level 4 spell?
Glyph Of Life (Life 3) – this spell seems like it ramps up its power very fast with additional life shards. Perhaps a base of +10 with +5 per shard is more appropriate?
Blood Curse (Death 4) – a game-changing spell. Can this be cast multiple times?
Pariden’s Return (no details) – this seems like a poor fit for any of the magic schools, really.
Mana Blast (Fire II and Water II) – this seems like a very dangerous spell to have for the AI, who can be stockpiling the mana like crazy at higher levels. It can easily one-shot any of the player’s heroes, which is not fun.
Mana Shield (Earth 4 and Water 4) – seems like another spell that can be easily abused. Perhaps make it a limit instead – e.g. for X amount of magic points spent, a buffer of “X / 5” temporary hitpoints are added to the unit, with X being a fixed value.
Soulburning (Death III and Fire III) – Not as efficient as fireball, but an interesting single-target killer.
Summoner’s Boon (?) – seems potentially over-powered.