She did it again!

The girl who made a racist remark at Louisa sometime back did the same thing again. While they were working in a group (they belong to the same group in class) today, that girl said 'brown people are not nice'. Then she went on to yi-ni-mi-ni-mai-ni-mo and pointed at Louisa saying 'you're the brownest of us all'. My girl tried to tell her (the incorrect way though) that another person is darker than dark. Adrian and I quickly explained to her that she shouldn't say that and that there are people of different skin colours in this world. Adrian then pointed out Aladdin, Mulan, Snow White etc to her. She had such a sad look when she told us this unfortunate incident (again!) I thought she was going to cry. I had been to speak to the head teacher before and I wondered if I should do it again. On one hand, I think this girl needs to be told to stop making all these insensitive remarks. By now I'm sure she's not ignorant and DOES know what she's talking about. On another, I don't wish to present myself as someone petty. BUT! If this goes on and that girl is being a bully here, not only will Louisa suffer from ill pyschological effects but in turn she may unknowingly be seen as a racist too (the remark she had said earlier). She'd told us she wants to be white (horrors!) on a few occasions before. I don't want her to think that being brown (yellow) is something she should be ashamed of and that she is any more different than others other than her skin colour. It is just not fair to be bullied because we look different. I tried to be gracious and invited that girl to Louisa's recent birthday party. I wanted to show her mum that I am not petty by including her. But it happened again. After writing all these, I think I have made up my mind. I AM going to speak to the head teacher again. Louisa's friends can say her drawings are not good, her jacket's not pretty enough or she can't run as fast as the others (I'm making these up) but I WILL NOT tolerate any remarks made at her regarding her skin colour. She may look different from most of her classmates but she's not stupid and is in fact in the best group of her class. How can I approach this matter carefully?