I remember one of my female friends who attended a very strict Scottish boarding school for girls telling me that the material used for making tawses had some conducting properties. This meant that the effect of swinging a tawse through a powerful magnetic field would induce a current in the tawse that would produce its own counter magnetic field.
Apparently a brilliant sixth form girl was aware of this and incorporated copper wiring in her knickers in such a manner as to be undetectable. The wiring was connected to a discretely situated power source.
The outcome was that there was a powerful magnetic field in the vicinity of the tawse as it made its way to the upturned derriere.
This would substantially reduce the effectiveness of the tawsing by a factor of at least 75%. This means a 12 stroke session with the McRostie would in effect be equivalent to 3 stokes, no problem for most senior girls.