Some are. There are spells that don't necessarily fit into easily-defined categories, so they go into more open schools.
Some of the basic spells types are easy to categorize. We all know what the main Illusions are-- the ones that affect our eyesight and create fake things we can see, or remove things we can no longer see. We know what Necromantic spells are-- the ones that involve dead bodies and the removal of health via the body's degradation (poisons, fatigue, disease.) Enchantment spells charm people and animals. Conjuration spells create objects out of nothing. Divination spells let us see things that we ordinarily shouldn't due to distance.
But what about spells that "create" energy in the moment with the purpose of just causing physical harm, and then it goes away-- or does it? Fireball is "conjured" out of thin air with the purpose of exploding but it doesn't stick around like a conjured object would... although the Fireball can light other object on fire which DOES stick around. So the fire stays, but the fireBALL doesn't. So what school would that be? The game thus made the Evocation school for those spells that create energy from out of nowhere and cause damage but do not remain-- although the results of the conjured energy might. Things might remain on fire... cold areas might remain and make things slippery or freeze something in place.
Or a spell like Cause Fear-- is it meant to be a charm that mind controls you into becoming fearful and thus should be Enchantment... or is the spell actually affecting the brain chemistry that produces the fear effect and thus they put in into Necromancy to go along with the other "body degradation" spells? Which school is more appropriate? Does it matter? And once we bring the Transmutation school into it-- the school all about changing the physical composition and properties of objects and creatures... it makes us ask why are poisons and diseases not Transmutations as they are changing healthy bodies into unhealthy ones?
And none of this even begins to broker the question about things like "Why aren't all the Lightning or Fire spells in their own Schools? Why isn't there a School of Pyromancy?" And the reason is because we can lump all these spells together into whatever categories we want, so the game just has to pick and choose the categories it wishes to give to us. But a lot of times those categories don't actually do a whole lot, and the ones we have are not necessarily consistent with what TYPE of categorization we are going for. Like the fact that Conjuration and Transmutation are about what the spells DO to objects... whereas Necromancy is about spells that affect a TYPE of object (dead things). So in truth, the School of Necromancy would be better served in a school system that was categorizing thing by the TYPE of things the spells were making, like Pyromancy (fire spells), Aeromancy (air spells), and Biomancy (living creature spells) etc.
At the end of the day... the Schools really don't need to exist at all, and they only do as a Legacy Sacred Cow to the Illusionist class of AD&D. The game wanted to keep that "class" in the game... so they created a school of spells to allow players to have it. And then they said "What other archtypical purveyors of magic are there and can we create schools for them too? Yep! The Necromancer! The Mesmer or Enchanter! The Summoner! And once they did that... it came down to creating the other schools to put all the rest of the other spells in... in whatever goofy category they could think of that would apply.
