Redskinluver:
Mouthsoaping for bad language is an old tradition in both homes and schools.
Okay it may have been used as an idle threat, or more likely by teachers as an expression of speech when scolding kids for bad language, but I've never seen nor heard of actual mouth soaping in USA schools. That said, most anything is possible in private religious schools.
Also, unless done very carefully mouth soaping could be very painful for the punisher if the punishee reflexively (or purposely) bites down at the wrong moment. Ask any pediatric dentist!
It's just a personal thing but... While mouth soaping might logically fit into the plot of a spanking story, for me there is no eroticism.