I think telling her it's 'the worst word you could ever call a girl' was unfortunate. Short step from that to believing that being a 'slut' (whether of the older or more recent variety) is the worst thing a girl can BE
Not necessarily. When I was in primary school, boys used to call each other "wanker", and I am pretty sure they didn't actually believe it was a bad thing ... to be honest, I am not sure they really knew what it means.
But, yeah, explain what it used to mean, what it means nowadays, and why you should never ever use it. Knowledge is power.
I had to actually teach my mother to not call me a slut. (She liked to use the term to voice her anger at the fact that I hadn't cleaned my room ...
)
If you bring it up with the teacher, there's no need to go all feminist, you can just pretend you're old-fashioned and a strict parent and don't want your daughter to learn dirty words.
Or, of course, you could go full on feminist and tell her that they usually don't use racist terms in books (or if so, discuss it in class) and that misogynist terms should be treated the same.