as far as I know, most girls who knew that they were going to be thrashed would not have the knowledge or ability to incorporate any type of protection into their knickers.
And even if they tried it would be too obvious. Maybe the original poster was confused and the incident happened in say 1990 when corporal punishment was still legal in private schools in the UK.
But even so, I think most girls would just accept a thrashing and forget about it. Remember in those days corporal punishment was relatively common even for senior girls, although I think that a good session with the McRostie was probably a little severe.
Never the less stranger incidents have occurred. It would be more likely the girl in question would have chosen the easier option of wearing say 4 or 8 pairs of thick knickers if she thought she could get away with it.