How old are the kids?
I really think that there are two kinds of smart mouthing - the "intend to piss Mom off" type, in which the kid has been thwarted in something and intentionally sets off to run her mouth and be annoying, and the "lacking filters type" type, in which the kid is just talking in a way that reflects her feelings at the time (generally annoyed about something), and totally not thinking about (or really caring about) how her words will come out and sound to others. Nine to twelve seem prime time for both of these, and really it's hard to tell the difference sometimes.
This so gets to me, btw, so I've worked on being extremely vigilant about my responses and not going off, and knowing that it's age-appropriate behavior - not fun, but normal. If I think it could be type 2 (and I try to give the benefit of the doubt), usually I go for a low-key, "That's not a polite was to speak to me." Usually the child (mine or one I work with) will catch herself then and rephrase or apologize, and then I know that it was inadvertant. If it seems intentional, then I think it's doubly important to keep myself in check, but my response will be stronger - "That was rude. Being rude to me is not okay."
I also think it's really important not to go tit-for-tat with this, and it's so easy to get caught up and start dishing rudeness back... but that perpetuate the cycle. Stay strong, be patient, it does slowly pass...
dar