Of course, nicely and revealingly bent over is the best way for a girl to take her punishment but history tells us that sometimes they were birched or caned standing as in this illustration, with hands fastened above.   See the post above for an excellent example.  Here is another:

In each case it is clear that the girl's bottom and not her back is to be whipped as her buttocks are bared while she is still wearing her blouse.