It is highly illegal to strike anyone – with the possible exception of children by their parents reasonably, not sure of the exact legal get-out on that one.
When CP was abolished in UK schools the repealing legislation removed the existing exemption from prosecution for assault where the “assault” took place in a school disciplinary environment. I’ll try and dig out the actual wording.
As far as indecent assault is concerned – it has to be proved that there was an intent to assault indecently. I remember one case up here in the NW papers around about mid-Eighties where the case for indecent assault against a Headmaster who smacked girls’ bare bums was halted when a House of Lords decision in a completely separate case ruled as I have said. The case against the Head was dropped because it couldn’t be proved that he was intent on indecent assault when he bared their bums. Just wish I could find the papers – it was in the Liverpool Daily Post and I think mentioned in the Times.