At my school the prefects did not really have the authority to slipper any of the girls but I once got a really good leg smacking from the head prefect. Whether she actually had the authority to do this or not did not seem relevant at the time, since the alternative would have been a visit to the office of the senior mistress for at least 6 of the best with the slipper plus a possible leg smacking as well. So I probably got off very lightly.
Most prefects did not smack or even hand out any sort of punishment. I knew of a few other girls that were smacked by the head prefect but I never knew of any to be slippered by any prefect. I was smacked privately in an empty classroom during the lunch break. I do not know about the other girls.