Ian Fleming wrote:
The fundamental objectives of HS English class should be to first teach students to write in a clear, concise, and grammatically correct manner and second develop their critical thinking thinking skills. In practice, most English teachers spend an inordinate amount of time attempting to teach bored students how to tease out meaning from dated novels with little limited context to their day to day lives. Most kids would be better served reading simpler novels so they can develop their ability to structure and write coherent paragraphs rather than grasp for meaning. All you need to do is read LetsRun to realize how poorly most people write.
My vote goes for Wide Sargasso Sea. Flaming turd of a book.
Reading great novels and learning to write are not mutually exclusive. Between homework and class time, a student is spending at least an hour per day on their English course. In that amount of time, one should be able to read a few thousand pages per year with plenty of time left to practice their own writing.
Your argument is akin to saying that kids shouldn't do sports because they should focus on academics. Well, a kid with the least bit of ambition can excel at both. Running a fast 800m never stopped me from doing well in the classroom, and reading Great Expectations didn't prevent me from learning to write. I'm glad I did all of those things.
Whether you become an English lit professor or a medical doctor, you're going to be reading challenging material all your life. There's no benefit to putting off challenging reading until undergrad.
Beyond the vocational side of learning, I think that a part of education should involve learning to enjoy what the world has to offer. This is why music and art classes are important. This is why athletics are a part of the educational system. It's also a damn good reason that great writing should be a part of the curriculum. Is every kid going to fall in love with Shakespeare? Of course not. But the kids who do will have their eyes opened to something that will bring them joy for years to come. While every kid won't like every book, hopefully every kid will discover something they enjoy through the process of trying out a number of different types of literature.
Anyway, by your logic, people who read a greater number of difficult novels in high school should be worse writers than students who read fewer difficult novels. I would guess that just the opposite is true.