There are so many techniques and concepts that can be taught in the scope of a year. Method books are are great at keeping classrooms moving in a linear fashion, with an unintentional but common flaw: When the ensemble literature performed has notes, rhythms, and many other concepts that are not in the book but still teachable.
Middle school builds upon the foundation of what students learn in their first years. D, G, and C major, along with A and E minor are the first scales students review we can use them with our literature to teach more complex rhythms, bowing, and finger positions. This is also the stage where vibrato can be introduced or reviewed.