The main "schools" posited in Legend really refer to different ways of accessing The Power.
Common Magic is the body of charms, cantrips and superstitions of the common people. In my Greentown setting, Common Magic comes from a variety of places - some Gods bestow them, some sorcerers release them as optimised forms of sorcery, rendered safe enough for ordinary citizens to use without the dangerous risks of sorcery; and some individuals with no magical ability otherwise just seem to come up with a Common Magic spell from some unexpected place: a dream, an inscription on a rock, or even from some place he goes to when he's been rendered unconscious by a Palsy spell to the head or some such.
Divine magic comes from the gods. Whether the gods are real or they're just a manifestation of the collective unconscious, the miracles come from that source and the only way to access them is through surrender to the will of the gods.
Sorcery is the magic of the force of the will, the intellect, the sorcerer's Holy Guardian Angel or True Word or whatever force majeure dwells within the human mind that can compel the universe to move at his beck and call.
Spirit magic (from the free Spirit Magic supplement) is another school - by tradition, shamans call upon no powers of their own, but for the ability to access and talk to the spirits and to call upon their powers and abilities.
And of course, there's Blood Magic (from Arcania of Legend: Blood Magic) - which isn't really a separate school, as such, but a different means of accessing the power, by expending one's own life force. Divine magic and sorcery both make use of this dangerous and volatile power. I'm sure some Common Magic and spirit magic could also use blood magic for a little extra kick, though it wasn't covered as such in Arcania of Legend: Blood Magic.
There is another way that Legend classifies magic: through the various religions, sorcerous orders, factions and cults present in the game's various settings. Legend uses the factions to restrict access to the more powerful spells and abilities to player characters until they are mature enough to use those powers wisely, in ways that would benefit the cult or order which teaches them. But those divisions are purely thematic - a sorcerous order themed around shapeshifting would teach Shapechange (Species) to (Species) and Dominate (Species) only because it's thematic to have that order teach those spells rather than, say, Animate (Iron) or the like. The Games Master determines the themes, making them appropriate to the setting; not the rules of the game.
