I have met this situation several times in my career. The trouble with education is it is infested with jumped up pox-doctors clerks who are of no consequence and simply enjoy their idea of being in power. Never let this kind of vermin stand in your way. To lack the human trait of compassion for one's others is simply to be inhuman. I always seek higher advice. Once a head said he would stop my pay while i was on active service with the army (TAVR & Reserves). He was formally cautioned by the War Office that any interuption to my out-of-school activities could result in him being charged with treason and then shot! Tee-hee. That shut him up. one deputy head even 'grassed me up' when he had seen me reporting for duty. Reassuringly good generals know their officers and truth always wins through!