It's not a "valid argument" to say a student would be prejudiced against a book simply because it was assigned in class. It's actually fallacious (specifically Dicto simpliciter fallacy, which is known as the sweeping generalizations fallacy). An argument is considered valid when it is backed up with proof, and the blog author's "valid argument" contains no proof at all.
Stating that 30% of kids finished a classic book but 90% finished the book when offered a well-written young adult novel is also fallacious. Readers are not given any more details about this "study" (it's implied that it was an informal study conducted by the blog author), we do not know how many teachers were polled, how many students responded truthfully, or even anything else at all except the numbers that could very easily have been fabricated (since your study lacked proof of their veracity).