One of the contenders for the best school. Honestly, I dislike playing against this school the most. Mines are just devastating against infantry units, especially in the hands of a mage with the Sorcerer skill. Wall and Crystal also cause a lot of problems for both infantry and ranged units. Phantom is a very annoying spell, especially if you don't have good offensive spells. The Hive, with its knockback effect, also causes many problems. There's no need to even talk about the usefulness of elementals and phoenixes. The only downside is that many of the spells require high spell power, so it might not be suitable for everyone.
Dark Magic
Also known for its various overpowered spells, such as Mind Control. If you don't have a way to counter it, you can lose because of your own army, especially orcs feel this on themselves. In general, it's good for weakening enemy units. It's worth noting the Disruption Ray, which is almost impossible to remove and can be cast until the enemy's defense drops to zero and their army becomes completely weak. Vampirism has further expanded the usefulness of this magic. Not only can you defeat many neutral creatures without losses, but it also counters various nasty effects: poison, fear, hornet swarm, plague, mind control, low morale. Another big plus is that this magic is less dependent on spell power. Therefore, it will suit any race and be useful at any stage of the game. That's why I don't know which to put first: Dark or Summoning.
Light Magic
A useful magic, especially for combat units. It makes it easier to defeat neutral creatures without losses, and it can also be used to effectively counter enemy magic, such as Dark and Chaos. It's also worth noting its usefulness during castle sieges, thanks to Teleport, and the somewhat overpowered Divine Wrath. It is also less dependent on spell power and will suit many. But I would still rank Summoning and Dark slightly higher.
Chaos Magic
This magic won't suit everyone. It's too dependent on spell power. In addition, although it's good at the beginning, its usefulness decreases over time. It's especially strong in the hands of a Necromancer, thanks to Irresistible Magic, Arcane Wave, and Sense Magic. But the others often have to think: is it worth taking? Yes, you can use Armageddon with a Dwarf, thanks to the fact that he has a lot of units immune to fire magic. You can take a Mage and enhance him with the Sorcerer skill and elemental gargoyles. You can also take a Ranger, thanks to Elder Druids. You can also take Necromancers with their spell power. But often the choice leans towards another school. So it's good, but overall it's inferior to the other schools. That's my opinion.