Although magic existed before the written word, it derives from well-known sources. The magic of Fire, Ice, and Storm comes from ancient Titans that ruled the world in the Days Before. The magic of Myth, Life, and Death come from the power of the mind, body, and spirit of the Wizard casting them. The magic of Balance combines all of them.
Each school of magic is very different and has its own philosophy, rhythm, and nature that combine with the elements of the universe.
No one is completely sure where magic comes from, nor how long it has been a part of the world. The only thing we do know is that magic comes from different sources. There are six schools of magic and one other school that draws upon the other six.
Site Credits: https://www.wizard101.com/game/schoolsofmagic