The irony is that you can’t change the magic schools for Haven without also changing the abilities of their units and the skills of their heroes. Light unicorns require light, and enchanted arrows and ballistae require chaos (or darkness as an alternative, but it would look strange for Haven). Neither chaos + summoning nor light + summoning works.
In general, it should be said that the schools in the game are reasonably well-chosen. 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.