Taming the tongue

My lovely daughter is not so lovely sometimes.  She's had some real interesting quotes lately for or about me.  I don't always hear these right from her, but from those she speaks to about me.  Here are a few of them:
1.  Some first graders stopped me in the hallway one day at school and said, "Are you nice?"  I said, "Do you think I am nice?"  They both smiled and said, "Yes!, but Isabelle tells us you are mean, because you always get her in trouble." - Thanks Izzy, but really you get yourself in trouble.

2. Izzy one morning before school (with a neck and eye roll)- "Mom, why are you always trying to be so pretty at work, isn't that like bragging?"

3.  A 2nd grader a lunch said to me as I was sitting next to Izzy at a table- "Are you her mom?"  Before I could answer, she interjects with, "No, she's my GRANDMA! HAHAHA!"

I am trying so hard to teach her to speak kindly about and to other people.  She has a sassy little mouth on her, and frankly I am out of ideas on how to train/ tame her tongue.  It saddens me that she will have to learn the hard way, by losing friends or having someone else do it to her and hurt her feelings.  I just don't know when my 5 year old became 15!  Where is the innocence, and the obliviousness to the mean and hurtful words in the world?  How do you teach her before it's too late, that joking is only a joke if the other person thinks it's funny?  At least right now, I am the one she speaks unkind about, but I want to stop it before it gets redirected to other people and mother's are calling me to complain.