Actually I believe some tawses in those days had metallic strips incorporated inside the leather to provide stability. If this was the case then the tawse would actually be considered quite a good conductor of electricity.
Also there would be no need to generate the magnetic field inside the girls’ knickers. As long as there was a relatively strong magnetic field in the room itself this would be sufficient.
This magnetic field could be generated using the existing wiring in the room by producing a power surge from some external source say a portable generator. however this surge might not do the school electrical system a terrible lot of good. Never the less it was worth a shot.