From the very beginning, I took the shortest path to the castle and captured it. That's why Isabelle couldn't reach her maximum potential. Moreover, I cleared half the map, and she still hadn't reached her maximum.
In my opinion, a Knight doesn't need to learn 2 magic schools. It's enough to learn Light or Darkness (depending on your preference). I prefer Light. It's better to upgrade some combat skills instead of learning a second magic school.