The notion that a wizard has a secondary school is a bit misleading. KI should really change this phrasing.
There really is no such thing as a secondary school unless you wear a mastery amulet. Are you wearing a Myth Mastery amulet? Otherwise casting a Humungo Frog is going to waste your power pips. Myth is, however, a decent secondary school for balance if you have a mastery amulet, at least for PvP purposes. In PvE, you probably won't ever need any attack spells from other schools.
If I was you I would probably learn Ice up to Tower Shields. Tower Shield can be a very handy defensive spell.
Another school that can really help out Balance is learning Death up to Feint. Combining a Judgement with a Feint spell can be really devastating.
And yes, you will eventually want to learn some Astral spells, but that's not until you reach Celestia. It seems that early on you never have enough training points, then at higher levels you usually have training points to spare. My level 90 Balance has 7 training points burning a hole in his pocket. Maybe the next world will have some new spells on which to spend the points.
Basically it says "Purchase the Amulet that corresponds to your secondary or tertiary school (not your primary school) so that Power Pips can be used for those spells."
So, for example, if you are Balance and Life, you would buy a Life Mastery Amulet. When you "equip" the Amulet (just like you do with a robe, hat, etc.), you can use Power Pips for your Life spells too. And when you are wearing a mastery amulet, you will see little stars or some kind of sparkle floating around your wizard. It looks really cool.
I would keep going on your secondary school being because what I did is have life as my secondary school and I keep going till i got to centaur and then i stopped an did astral and now I am just waiting for a training point and a better spell to get instead of that spell that does +65 or something for more life