I would divide the schools like this (most of this correlates with Heroes 3):
Fire
Armageddon, Berserk, Blindness, Fire Wall, Fireball, Immolate, Curse (Weaken), Retribution

Air
Arrow, Dodge, Bolt, Haste, Lightning, Slow, Chain Lightning, Teleport, Phantom, Hornets, Hive

Water
Dispel, Blessing (Divine Strength), Weakness, Blur, Ice Block, Ring, Cold

Earth
Stone Skin, Slow, Anti-Magic, Raise, Resurrection, Meteor, Thorns, Shock, Plague, Unholy Word, Swords, Fist

Vampirism - water or earth (water).
Grief - water or earth (water).
Crystal - water or earth (water).
Holy Word - air, water, or fire (need to look at the visuals) (fire).
Shield - earth or air (depends on whether the visuals will change) (air).
Mines - earth (but without fire ones) or fire (fire).
Regeneration - earth or water (water).
Phoenix, your elements can be replaced - as you like, let's say fire.

In parentheses, I put the default options that correspond to an even distribution of spells (with the exception that there are 11 spells in fire and 13 in water, but the hive can be redone into some kind of totem that spits fireballs.
You also need to look at the animations (visuals), perhaps some rearrangements of low-level debuffs and buffs.

I will not distribute them by levels, because this is insignificant against the background of introducing a new elemental system (with the exception of mass buffs and debuffs), any spell can be easily adjusted (balanced) to any level.

For example, moving Phantom and Teleport to water. Based on the hive, you can assemble a totem that spits fireballs. Or draw the fist in fire, and in this case, move regeneration to earth. In this case, it may be possible to achieve an even distribution of spells.

I did not indicate Earthquake, because its presence is not necessary at all (situational).

Bolt - definitely air.
Dispel - definitely water.
Swords - earth, or change the name and visuals.
The rest sometimes differ from the classic (Heroes 3), but you can get used to it.