For a grade 9 student, I would recommend Dr. Gary Sheffield's A Short History of the First World War. It's brief, as the title implies, but it is very much up-to-date which is not what can be said about many other general First World War books that people point towards (such as Keegan's one volume work). It is also a good introduction to the conflict as well as provides an overview that encompasses the entire war instead of focusing simply on the Western Front. From there, your student will hopefully find something specific that they find interest and can therefore branch out from there. Please avoid Barbara Tuchman's well-written but wholly outdated Guns of August. The scholarship of the First World War has come a long way since.