Your employer seems like a (expletive deleted). Since he isn't playing fair with you I don't see why you should play fair with him.
What are the possibilities of shifting camp to to another school - along with with your students? If not geographically, then administratively ie operate under another flying school licence but on your current aerodrome?
Don't really know what the UK rules are on this but it would give the appropriate message to your current employer.
But get out of there regardless.
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Don't mess with the red switches.