P.O. stands for Pledge Observed and in the simplest terms means you are upholding The Gunnery honor code.
The Gunnery operates under a strict interpretation of an honor code that is especially applicable to academics. At the beginning of the academic year, every new student is required to write out and then sign the following pledge:
I have neither given nor received improper aid in the preparation of this assignment. Unless properly attributed to others, it represents exclusively my own work.
Plagiarism and other forms of academic dishonesty such as cheating are considered major school violations. Students must initial each assessment and assignment they submit, indicating that the work is their own, and must attribute sources as appropriate. This reaffirmation of your adherence to the academic honor code is referred to at The Gunnery as “pledging” your work. Whether or not you remember to pledge an assignment or an assessment, you are still beholden to its obligations and responsibilities.