The irony is that you can't change the schools of magic for the Elves without also changing the abilities of their units and the skills of their heroes. Light Unicorns require Light, and Enchanted Arrows and Ballista require Chaos (or Darkness as an alternative, but it would look strange for the Elves). Neither Chaos + Summoning nor Light + Summoning works.
In general, it should be said that the schools of magic are reasonably well-chosen in the game. They don't always fully match the lore of the factions, but the current arrangement of schools provides the best synergy, and that's more important.
So, Light + Summoning is perfectly fine for the High Elves.
Chaos + Darkness works well for the Dark Elves, as Darkness can be used for Subjugation. Very often, in the RTA (Arena) format, Dark Elves are developed specifically towards Darkness.
Finally, Mages often play with both Summoning and Chaos, but Light doesn't suit them at all.
Added after 44 minutes
Humans - Light, Darkness.
Wood Elves - Light, Summoning.
Mages - Summoning, Chaos.
Dark Elves - Chaos, Darkness.
Necromancers - Darkness, Summoning.
Demons - Chaos, secondary buffs and debuffs.
Orcs - Shouts, Sword Magic, which is enhanced Warrior development.
Dwarves - Light, Chaos. (They shouldn't just throw runes).
And everything seems to be fine.
Only for Demons, it's still Chaos and Darkness, with an emphasis on Chaos, and for Dark Elves, it's Darkness.
