They would be related to a group of spheres, preferably, and the effects in a school would be dependent on those spheres. Some schools would probably end up with overlapping effects. I was going to stick fire, ice, and magma/volcanoes into one school, another that revolves around storms, so it might include thunder, lightning, and water, water might be shared with oceans and fish, which shares aquatic transformations with a polymorphy school that also overlaps with elven druid plant and animal worship that acts as a foil for dwarven mountains, alcohol and caverns. So on and so forth.
The idea is that you could learn magic in general, which gives access to a lot of common low-level effects, some advanced effects, and specific schools give access to something in between that level and secrets, and have more advanced and mastery spells that approach secrets in power. This would end up coinciding with making the general magic effects weaker and secrets much stronger, on average.
You can study specific schools via their associated spellbooks, probably needing some reagents available in environments or beasts associatable with those spheres that make up the school, could be found in the sites owned by civilizations that practice those schools, or from a wizard of a caste that practices that school of magic. So it would also coincide with wizards having multiple castes, with abilities and what they can teach dependent on their caste.
That's the rough plan anyway. It would be a lot of work and if I got it out all at once it would probably mean a long gap in development.
Anyway, 4.07 update has been posted to the download page. New spheres include Thralldom, Fate, and Deformity, and there were some fixes related to discussion in this thread and my own playtesting.