Presumably this is going on at lunchtime so your daughter can go elsewhere with different friends if she is uncomfortable. However, if you are upset, you could speak to your daughter's tutor or another member of staff.
As others have said, a banned book (or banned anything) can be extremely attractive to teenagers. This reminds me of a recent blog post that I read by a literacy specialist called David Didau. Here's an extract. I think the part about World Book Day is fantastic!
"One librarian I worked with stands out. Toby Dyer was an incongruous figure. For one he was young – in his late twenties – and for another he was cool. Much cooler than any librarian has a right to be. He had a tattoo which read, “Reading Is A Rebel Act”, a sentiment that rang deep within – this was how reading always felt to me as a teenager! He made it his business to imbue books with edginess and danger. He engaged in what he called ‘reading terrorism’ – bursting unannounced into classrooms, reading a passage of prose or poetry and then dashing off, cackling maniacally.
"I met up with Toby last year. During our reminiscences, he told me about a reading assembly he’d given one World Book Day. He took the stage, his face a thunderclap, wielding a copy of Kevin Brooks’ The Bunker Diary. Although this book won the Carnegie Award in 2014, it’s probably one of the most controversial children’s books ever written. Anyway, Toby told his audience that he’d caught a year 8 boy reading the book and had confiscated it. He told them some of the more salacious details and said that the school absolutely could not endorse such filth. He said he’d heard there were other illicit copies floating around the school and that he was running an amnesty in the library: if copies were handed in before the end of the week no further action would be taken. Apparently he got the deputy head to go along with him. Anyway, he left his ‘amnesty’ pile lying around, unsupervised in the library and by the end of the day, every copy had been borrowed. The book became the most talked about reading phenomena since Harry Potter first hit the shelves."
www.learningspy.co.uk/reading/on-libraries/