I would divide the schools of magic like this (most of this correlates with Heroes 3):
Fire
Armageddon, Berserk, Blindness, Firestorm, Fireball, Judgement, Curse (Weakness), Retribution

Air
Arrow, Dodge, Ray, Haste, Lightning, Control, Chain Lightning, Teleport, Phantom, Hornets, Hive

Water
Dispel, Blessing (Divine Strength), Weakness, Confusion, Stone, Ring, Cold

Earth
Stone Skin, Slow, Antimagic, Raise, Resurrection, Meteor, Thorns, Shock, Plague, Unholy Word, Swords, Fist

Vampirism - water or earth (water).
Gloom - water or earth (water).
Crystal - water or earth (water).
Holy Word - air, water, or fire (need to consider the visuals) (fire).
Shield - earth or air (depends on whether the visuals will change) (air).
Mines - earth (but not fire mines) or fire (fire).
Regeneration - earth or water (water).
Phoenix, Elements can be swapped - to taste, let's say fire.

In parentheses, I've 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 redesigned into some kind of totem that spits fireballs.
Also, you need to look at the animations (visuals), perhaps some rearrangements of low-level debuffs and buffs.

I won't assign levels, as this is insignificant against the backdrop of introducing a new elemental system (with the exception of mass buffs/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 create a totem that spits fireballs. Or draw the Fist as fiery, and in this case, move Regeneration to earth. In this case, it may be possible to achieve an even distribution of spells.

I didn't mention Earthquake because its presence is not necessary at all (situational).