It is difficult to imagine a time in history when a male teacher spanking a girl pupil would not have been regarded as sexual abuse by every right thinking person in the land.
However, there seems to be abundant evidence to show that it happened in the UK in relatively modern times and still happens in the US and that it met/meets widespread acceptance (at least among those who regard CP itself as acceptable).
I have never heard from a reliable source of a case where a teenage schoolgirl was in modern times and in accordance with accepted procedures punished bare in the UK.
Also, no male teacher in the UK has ever spanked a female pupil’s bare bottom without subsequently facing criminal charges for assault – although amazing as it may seem the actual act of spanking her on the bare was not, in itself, illegal.