Grade C is supposed average for teachers if you don't screw up. It's also hard to go beyond C+ unless you are given some specific duties or high profile posts in the school. Its sad but teaching is seriously less valued than your performance in other areas (unless you come up with some teaching program that can be applied to every class rather than your own students). You need to volunteer for duties or aim to be in the CCA/Comms which are more highly valued (harder work but better rewards). From experience I have realised that if the principal values you highly, its easier to get a better grade. (although you still might have to take up position to earn it. Hey its better than taking up the position and still not being recognised right.)

In the past I had one motto for my teaching career. "MEME" Minimum effort Maximum Exposure. Sad but teaching is sometimes like the army. Super Wayang kind. If you look like you are doing alot (which you may or may not be doing), you get rewarded for it even if others might be doing more without being noticed. The whole performance grading is so subjective that I've given up thinking about it. Life is never fair and harping on it only makes one feel miserable. I teach because I like interacting and guiding students so I figured anything else is a bonus.