Most spells require skill in two different schools. The others require three different skills in order to cast them.However, just because a particular type of magic is required doesn't mean that the level of that magic effects a particular quality of the spell. Most spells have parameters based on at least two schools, but some only use one. Only two of the spells that require three types of magic actually reflect the level of skill in all three schools. Some of the spells aren't affected by skill level at all, just rank.
