Leg slapping for girls was introduced at our school with the arrival of a new Deputy Headmistress who clearly had a penchant for such discipline. Back in the late sixties as this was, white knee socks were still obligatory for all except sixth form girls and with school skirts at the time becoming ever shorter, there were plenty of conveniently bare legs about the place should punishment be required.
Since it was a mixed sex school, girls were only ever dealt with in private there was an ante room near the school office which became synonymous with tearful girls and sore legs. Twice I suffered this punishment once for being caught outside school during breaktime and once for answering back a prefect who duly reported me. Cant be 100% sure but I guess I was about 14 the first time and 15 the second.