I suppose I'd check whether the job description (as surely all do?) includes the "anything else asked of you by your manager/headteacher" line, and then let them know that this is a reasonable additional task that will be part of their role on an ongoing basis. On a more placatory note, I'd probably want to meet with them as a team and find out why they think this is a problem - usually that kind of resistance is a symptom of underlying discontent, so trying to find out what the issues are generally and dealing with those (if possible) might be a way of getting them onside for this and all other aspects of their work.

In reply to Nina Waters:

Thanks for your reply. Appreciated. Yes, we do have that line in the job specs.