I agree with aurynne, she was a guest, so didn't muck in.
She's a student so is skint, and she 19 so a little inexperienced and gormless. It's not easy having to stay with your uncle and his his wife, ie you!!
She does sounds a bit up herself talking about inappropriate things in front of your kids, but back in the day, even I was asked not to swear in front of my sisters kids when I was 21. I just did it without thinking. Now I look back and see myself as a gormless twit.
My sister's kids would eat you out of house and home when they visit, but they live on bread and cheese at college, saving up to go to Venice and sofa surf, so their priorities are just aligned that way.
The thing is OP, you don't like her do you!? So you're giving her a bad name and you plan on hanging her!! And fair enough if that rocks your boat. But she'll probably come round when her brain has finished growing.
I also don't think this thread is a public / state school thing. It's a 19 year old thing, and a guest / family boundary issue.