Just say something like
"Do you have any idea how annoying that is?" in a really serious tone when everything is quiet in the class, preferably right after she says something offensive.

Follow up with
"I expected better from you...". Make absolutely sure you do it when everything is quiet and the laughter has died down; say it plainly and clearly so you don't have to repeat yourself and look like a dumbass.

Facial expression is important, don't wear a goofy smile while you say this, try to look as serious as possible, and afterwards don't say anything to her, just go on about your day. Believe me, she will have that in her head eating away at her for the rest of the day.

What this does is make her seem like another really annoying kid that no one likes because they have to mock everyone else to be cool, so it helps that you seem like the victim who just wants her to stfu.