Isn't that a conversation to be had with your headteacher? I can't imagine personally agreeing to fund someone learning the piano a part of CPD, but as a secondary head I've always got pianists in school if I want one. A primary head might be willing, but I doubt they'd fund it if you're starting from scratch, i.e. below about grade 5. It would take too long, would cost too much and there's nothing to stop you leaving during the process, having given nothing to the school.