Unless the principal was also a teacher for one of your core courses in 10th or 11th grade, his letter does not meet the requirements. However, he could be an excellent extra recommendation if he knows you well in a context outside of the classroom and can speak to qualities that your teachers might not know about. (I assume his letter is not a substitute for the guidance counselor’s letter, which is required, right?)
If that’s not the case, you need to find a soft way to deflect him. It can be awkward when someone in a position of authority wants to ‘help’ by providing a recommendation that isn’t wanted or needed but you can explain the restriction to them and ask if they would write the rec and keep in ‘on file’ in the event you need it for a job, scholarship, or grant interview. You never know when an extra ‘rave’ might come in handy.