opb:
particularly if the spankee doesn't know up till that point that the spanking is to be on the bare bottom, and harbours hopes that they might get away without the humiliation or additional pain involved.
Yes, I agree. Usually the schoolmistress or whoever she is, will say something along the lines of:
"I am afraid your behaviour leaves me little choice. In this instance I shall administer a severe punishment on your bare bottom."
The enjoyable thing about this scenario is the mild hypocrisy involved. Of course she is not in the least bit afraid, she is in fact, highly delighted she has an excellent excuse to smack a bare bottom.
It would be an interesting exercise to write a spanking story and avoid all the cliches. I think you might have quite an unusual story at the end.