pamish, the book being read had not passed me by at all, or any other poster as far as I can tell. No need to shout or talk down to us.
I got into reading Agatha Christie at 11 or 12, so same school year as OPs dd because we were asked to write some piece of homework from the pov of a detective. I don't think it was even English homework. I'd heard of Miss Marple but didn't actually know anything about her, so I found an AC story featuring her, read it and wrote the homework over a weekend.
No harm done then and I'd say the simplistic, formulaic style of AC's writing was well suited to my 11yo immaturity. (I moved on to Dorothy L Sayers and all sorts of adult fiction by 13 or 14). I also think it was entirely a good thing that when, at 13, we read Animal Farm, I moved straight on to 1984 and other Orwell.
I could easily have taken this apparently flabby, ill-defined homework task as a reason to read anything at all by McEwan and, at 11, while I wouldn't have come to any harm, I think it would have been a foreseeable consequence better avoided.
That is what I'd be saying to the teacher.