I agree that the slipper can be quite effective, but in most cases not really necessary. At my school the slipper was not used in the classroom but only privately, mainly by the senior mistress. Most classroom teachers did not need any implement to control the class. Admittedly it was a girls school, and generally speaking most girls were and are still very well behaved.
I think that the slipper was being phased out by about 1990. I cannot remember any girl being slippered at my school (private) after that.
I agree with Esmerelda.It was not really required since a trip to the front desk was usually sufficient to settle most girls down. It would be interesting to hear from any girl who claims she was slippered after 1990.