neliz87:
I have read one story that matches your scenario: integrity by Crimson Kid.
Don't know if you already read that story but it is the only one I can think of.
Thanks for the 'plug,' that one was originally written for a short story contest with a 500-word limit.
My story "Positive Reinforcement" would also fit the description, but the spanking action is merely auditory as it's being overheard but not directly observed...

--C.K.