I've probably played as each school, But wound up deleting at least four of them, so I was thinking. Maybe the Astral schools should be playable. Sun, Star, Moon.
They are when you get to the right level. Then you learn the spells but, frankly, they're not strong enough to support their own Ravenwood school. Look at their spells. They're mostly things that enhance your school spells or polymorphs. How could be possibly be stand alone schools given that?