So what if you shift gGeo's idea slightly:
1st point in first, second, and third school: 1 point
1st point in fourth school: 2 points
1st point in fifth school: 3 points
...
So you are encouraged to have a little diversity, but it grows more difficult if you spread out too much.
-OR-
You can only ever have points in 3-4 different schools, but you are able to sell old points back and respec. This might incur some cost or have a limitation or something so you can't constantly be respeccing, but it does allow you to grow and change over the course of the game.
