Edit: Also I am getting around to actually putting my magic spell list together. How do you decide to separate spells. Like say you have a spell that can damage the dead and lean towards holy but is a fire magic. It has the potential to be in two schools or colleges. Or how about a potential spell that would fit both necromancy or shadow magic? I'm just looking for a way to divide my magic before dividing it into class and racial systems.
There's nothing wrong with a spell being in two (or more!) categories. Maybe a simple mending charm for a ripped shirt works equally for stitching up a deep wound, for example. So should we teach it Healing or Home Economics? I would say, teach it in both.
Alternatively, here are some ways you could sort them:
BY TYPE: spells that create (create fire, create food), spells that destroy (remove a zit, kill, i.e., destroy life), spells that alter (make something grow, make something shrink, make it turn purple), spells that restore (mending charms, healing charms).
BY FEASIBILITY: (A) magic used to do something a person could do if they were tall enough/strong enough (push a boulder away); (B) magic used to do something a person could do if they had the right tool (create fire, kill weeds); (C) magic used to do something a person could not by any other means do (revive the dead, read thoughts)
BY CREATOR
So what if that slicing charm is really good at severing your enemy's head from his neck? It belongs in Home Ec because Matilda Prunewhistle originally developed it for carving Halloween pumpkins.
In this scenario your potential spell that could fit necromancy or shadow magic (skiamancy/gnophomancy if we want to be Greek) could be classified under necromancy because the guy who came up with it was, you guessed it, a necromancer.
BY MOST USE
As the title says. In what field is it most used? Stick it there.