Not a novel, but The Merchant of Venice is excellent ninth grade reading.
I read that in 9th grade. And Great Expectations.
My point was that there isn't a play or a volume of poetry on the list. By anyone. The omission of Shakespeare is no more odd than the omission of George Bernard Shaw, Oscar Wilde, Tennessee Williams, Arthur Miller, and so on. I haven't been able to track down the original list from the Grauniad, so I don't know the wording of the question, but "book" in this context appears to mean "novel" (with some wiggle room between fiction and non-).
No disrespect for the Bard, and no diminution of his importance to English literature and culture; this just isn't his list.