Everyone should train the Sun damage enchantments all the way to Colossal as soon as possible. It makes your attacks much stronger at zero cost.
Star spells can be useful too, but which depend on playing style, your primary school, and your orientation -- PvE or PvP. When you get to Azteca you can learn more powerful Star damage aurae and Sun boosts for blades and traps. You will want those.
Moon transformations can be fun, but aren't needed, so most players skip those.
I agree with Prince. Sun attack boosts are the most useful to every school and will be used in pretty much every fight. Star auras are useful in some PvE situations but used much less often. They have more uses in PvP. Moon spells are pretty much only useful in PvE team fights where you are missing a healer or tank, the Life and Ice spells. Even then they aren't all that useful for long.
My suggestion is to train all the Sun damage spells, potent trap, and sharpened blade.
From Star, train empowerment if your character is a tank from the Death, Ice, Balance, or Life schools. Also train your school's damage aura in Azteca. Buy treasure card versions of everything else.
Don't bother training anything Moon school unless KI makes the spells better. Right now, they're just not very good at all. They all make your stats too weak.
By the time you reach level 100, gaining a TP every five levels, and based on how many of Zeke's quests, side quests etc. you have done, you will have earned 40+ TPs, so don't feel like you won't have the opportunity to learn more spells from other schools Steven.
Once you're able to venture in Celestia, the game changes drastically and you will want to have a small surplus of TPs at your disposal for the Astral school spell options.
Please be aware that some of these spells have pre-requisites, such as level or learning the previous version of the same spell, so you will use up your TPs quickly. Proceed with a bit of caution to avoid unnecessary trips to Golem Court for a re-train.
I stayed away from the Moon schools completely, simply because I wasn't interested in becoming a gobbler or a cat. Agree with the other replies re: potent trap, sharpened blade, and colossus. The specific school boosts (for ice, it's called Sleet Storm) are fantastic.
So unless transformations are your thing, I would focus on your Sun and Star spells.
Hard to say, but definitely between star or sun. I see practically EVERY wizard use the star spells that increases or decreases incoming with a aura. But on the other hand, many wizards do use sun for enchanting..
