I have a death wizard and an ice. Neither one has a secondary school.
As you progress in the game, it becomes harder to cast out of school spells unless you have a mastery amulet. Most gear gives damage, accuracy, critical boosts to your own school making it impractical to use out of school attacks even when fighting same school bosses (that is what converts are for).
Here is how I used the training points on my wizards. Both are level 95.
Death: ice to tower shield, spirit blades and spirit traps from Niles in krok. The sun school boosts up to colossal. Star school spells of vengeance, fortify, and amplify, and virulence. That left me plenty of training points for the shadow spells I will eventually get.
Ice: death to feint, life to sprite, elemental blades and elemental traps from Niles. Sun school up to colossal. Star school spells of vengeance, amplify, fortify, sleet storm, and berserk. As with death, I still have plenty of training points to use later.
One thing to remember, just because you have training points, doesn't mean you have to use them right away.