Girls may be much the same as boys but I have no data on this from my school days as girls were not caned. They rarely committed canable offences and were, in any event, consider much too delicate to be caned. There were no Jennys around to defend their rights.
It is difficult, if not impossible, to give an objective measure of pain. From my own experience I would say that two strokes hurt twice as much as one, but three strokes hurt twice as much as two. It is at the third stroke when the pain seems to really take off, every additional stroke becoming increasingly hard to bear. As I have written elsewhere recently, one of the factors is how long the extreme pain lasts – that is why a longer interval between strokes makes the caning feel worse. Then, you feel the shock of every stroke separately and instead of the pain declining, it is constantly re-ignited