@Minigiant
These definitions for each Spell School derive from the bottom up from the mechanics and themes of D&D itself.
Go thru every spell in the game (and every feature of every class, every race, and every feat) and group them together by theme. You end up with these categories.
Notice, that Abjuration is missing because it is unlike the other themes. It is an end rather than a means. The other themes are means. Abjuration is a tag that indicates purpose. For example,
Cure Wounds 1st-Slot Transmutation (Healing, Abjuration),
Alarm 1st-Slot Divination (Scry, Abjuration),
Antimagic Sphere 8th-Slot Conjuration (Dweomer, Abjuration), and so on.
Broadly speaking the themes are Mind (Enchantment and Illusion), Matter (Evocation), and Life (Transmutation), plus Spirit (Divination and Conjuration), plus Nonmagic. Then there is a kind "Anti" theme (Necromancy as anti-mind, anti-matter, and anti-life).
Even the Nonmagic forms Mental (Knowledge and Persuasion), Physical (Weapons and Equipment), and Life (Athletics, Survival).
The Spell Schools correspond this deep structure of the D&D game.