I don't really think of any school as my "secondary". Regardless of what school my wizard is I always train to Feint, to Satyr, to Tower Shield, to Fire Elf, Blades/Traps and Reshuffle, Damage boosts and Sharp/Potent, Various auras depending on school.
I still have 2-3 training points left over on all. I wouldn't call any of those schools my secondary. I use them as support schools. I don't think using outside school attack spells is really worth it, except in certain specific cases (clearing shields/weakness, keeping Belloq from cheating...) Your Primary school spells will always be better because of gear boosts.
As a balance wizard, I consider death to feint essential, and once you get high enough you want all the sun damage boost spells. Everything else is optional. I will say training life to get healing spells seems silly when you get availing hands for free.
I do feel all wizards should figure out a way to shield against any damage type (except possibly balance, since tower shields are so hard to train). In addition to my elemental and spirit triple shields, I also have trained the 80% individual shields for death, fire, ice, and storm. I also have the myth/life double shield as a prerequisite for feint, so I have foregone training individual shields for myth and life. This gives me two different shields for every damage type (except balance).