Unfair faculty performance evaluation

Hi.  I am faculty member in the high school department in an SUC here in Manila.  For three years now, I have been receiving relatively low performance evaluation from our principal although I would receive higher ratings from my student evaluation. The contents of the evaluation is based on my actual teaching, and since my principal has only observed my class once in the entire 3 years, I have been questioning the merits with which she based my rating.  I have also been active in supporting activities, and would volunteer for activities like proctoring in entrance and aptitude tests, and screening incoming students despite the fact that I am no longer required to render such as I am designee.

Recently, I just realized that the very low rating she gave will have grave effect on my NBC promotion as my CCE rating is high but if my QCE rating is low, I will not be able to enjoy my promotion.

I am planning on writing our VP for Academic Affairs to report this unfair evaluation practice, but I hope you can help me cite a clause from the Civil Service or our constitution to help strengthen my complaint.  Thank you very much.