When I was in eighth grade, the focus of my English class was diagramming sentences. The teacher took it very seriously, teaching the subject as if it were one of the wonders of the world. Unfortunately, she taught it as an end in itself, unrelated to the writing process. Further, the most extensive writing we did in her class occurred occasionally after class. As she was also my homeroom teacher, whenever anyone in the class received a demerit during the previous week (I believe that was the timeframe), she made us all write some inane sentence a few hundred times.
Diagramming sentences, however, can have educational value when it’s integrated with the writing process. The book that’s my guide to the subject is Grammar by Diagram by Cindy L. Vitto.