You should go back to your principal, along with your parents and your previous reports. Tell that you need to monitor sugars and hence need the monitor back. It wil not be bad. Your principal will understand the need and wil help you. Do not use the monitor at school, only when absolutely needed, like you feel you are getting your sugars low, or high and need to check, that time take permission for two minutes from your teacher/lecturer telling you are feeling uncomfortable and need to check sugars, and then check it. In this way you will avoid other unnecessary issues.