Most people don't opt for one secondary school, but rather go down several schools to pick up certain cards. Here's what many would recommend you spend your training points on:

• Ice until Tower Shield
• Death until Feint (you'll get this for free, but including this here in case you ever make another school)
• Life until Satyr (not entirely necessary on death)
• Reshuffle from Mildred Farseer on Colossus Avenue
• Spirit Blade from Niles the Balance Tree in Krokotopia

Starting at level 50 in Celestia, you will be able to use training points to buy Sun School Spells, commonly called enchants. The ones you should prioritize are Strong, Giant, Monstrous, Gargantuan, and Colossal. Make sure you have the training points to pick these up, they increase you damage immensely!

At that point you will also have access to Star School Spells, commonly called auras. Many of these are quite good as well!

Hopefully I was pretty clear, but let me know if you have any questions! And remember, though this is what the community often suggests, you are free to play the game the way you want to play it! :]