As I said in my post to "The Punishment Must Fit…” thread, in girls schools it was used in cases where, according to you, it would be unlikely even to work with boys. In boys’ schools, it was used in cases were it was most likely to work. It’s wouldn’t be surprising if appeared to be ineffective with girls – for that reason. If it had been used on girl the same as it was on boys, it might have proved just as effective – and vice versa.
True – except there’s also research on its use in families (more of it in fact) where corporal punishment was actually used slightly more readily with girls than with boys. And different results were seen.