I suggested, above, that a defence to a charge of battery would be available to a headmaster who had been given permission by parents to use CP on their child. I think, though, that my claim that a prosecution would fail was a little hasty: the prosecution could fail, but it would do so only if the defence succeeded!
Now that depends on the facts of the matter. It is not automatically the case that an action taken in work time and on work premises is undertaken on behalf of the employer: for example, just watch an employer deny responsibility when someone is caught downloading child pornography on a work computer in work time.