...and it doesn't much help that schools of magic appear to have been defined by one person and then assigned to spells by another person while those two were not at all on the same page about what those definitions were, so there have always been spells which seemed to have the wrong school, and those have frequently been changed around, and even now we still have misapplied schools (even after errata changes the school, as some healing magic was given the conjuration school [normal for 3e, despite that having not made sense] and errata changed that to evocation [normal for 5e, despite that also not making sense] while the necromancy school has constantly been defined as manipulating the energies of life and death)
Yeah, the schools of magic in D&D never made much sense. They started in 1e as basically pointless bits of flavor attached to the spells (I don't know if 1e had any magic items or the like that differentiated between magic schools, but there certainly wasn't much effect to them). In Dragonlance Adventures, they were used to differentiate between the different Orders of High Sorcery, each of which could use a subset of the wizard spells based on schools.
2e was the first edition where schools actually had some mechanical weight. They integrated the Illusionist spell list into the Wizard spell list, and gave each school their own specialists. This did not work so well, because the schools had never been designed to carry that sort of mechanical weight, and they were quite unbalanced (e.g. Necromancy had a single 1st-level spell,
chill touch, while Alteration (the previous name for Transmutation) had 17). I understand it also made a lot of grognards unhappy, because the Illusionist lost a lot of its identity when their distinct spell list became available to generic mages. Spells like
blur,
improved invisibility,
minor creation, and
phantasmal killer used to be exclusive to Illusionists.
3e and later 3.5e made some advances toward balancing the magic schools, and gave some mechanical weight to the schools themselves (e.g. using the sub-schools of Illusion to differentiate between a spell that creates an actual image, one that changes the way something looks, and one that makes someone
think something's there). However, they never fixed the underlying problem, and that is that the schools don't really
mean anything. They're not based on anything in particular, which can be seen in things like the confusion about where healing spells belong (for the record, using the 3e definition they were fine in Conjuration, because they were based on channeling positive energy). They started out as meaningless labels, and that's pretty much where they've stayed.
As a contrast, look at the Color Wheel in Magic: the Gathering. Those colors actually
mean things - both when it comes to methods and goals. Red deals with fire, lightning, speed, emotion, recklessness, and inventiveness, and it primarily desires Freedom. Blue deals in trickery, ice, illusion, flight, transformation, and it primarily desires Knowledge. These are strong identities, and it's fairly easy to tell the difference between a Red and a Blue mage. But it's not that easy to tell the difference between a D&D evoker and a diviner.
