like the poster above me said, it's in the order you learn them. You do not need to buy the spell again if you remove it though, it just gets put into your inventory, so you can learn them in any order you want.
Blizzard Fire Sleep
Poison Blind Thunder
Blizzara Fira Thundara
Blizzaga Break Shade
Raze Erase Thundaga
Bio Firage Warp
etc...

I arranged it that way by unlearning spells and putting them in the order I wanted

THE JACKEL