Most people don't train other schools as true secondary schools they manly do it to target single utility cards.
most popular across the board:
Satyr/Sprite heals and myth/death shield.and occasionally Sanctuary.
Feint! and the life shield (as you will likely never secondary into myth unless you seriously want earthquake) Feint is big for all mages.
For the hefty fire/thunder shield and the all important Tower shield (this is the only way to block balance and is great for quick decks needing a bit of shields) and saves gold on tower treasures in pve/pvp.
Some will also grab weakness as its super early but most will just get reshuffle (colossus boulevard trainer) or the 1 pip trap/blades (Nigel trainer krokotopia). You are balance so this doesn't apply as you get these for free.
The reasons for this is it takes a mastery necklace to really use a secondary school and even then your gear wont really support dual class unless you craft it that way (which often sacrifices other important stats).
other than that sun (the damage line mainly) and star are usually best for training points as moon while nifty is a bit lackluster.
now note this is efficient training point usage. not everyone's fun is tied to max efficiency do what works for you please.
Honestly, I would suggest that Death up to Feint is a better choice. Whether we're supporting someone else or hitting on our own, having that extra 70% of damage in our spell deck makes the battle end much faster.
As for Tower, the reason I think it's a waste of time for Balance is because we already have the triple shields that cover all the schools (other than Balance).
It's generally worth it, as it is an extremely versatile spell. If you have the extra training points and no other spell priorities I'd say go for it. Especially so for you because a Balance/Life combo is already a strong support, and combining your Spirit/Elemental Shield with a Tower Shield makes for a great defense. However, if you want more offensive-based support, consider training Death to Feint. Feint is super strong and makes later parts of the game much easier.