Sentence Fluency is just one of the six writing traits. In Nevada, where only four traits (ideas, organization, voice, and conventions) are "officially" assessed on the fifth grade state writing test, you can elements of sentence fluency embedded on the state's conventions rubric. So, as we say in our trait trainings, even though sentence fluency doesn't have its own rubric on the state test, teaching it well can improve students' convention scores.
Sentence Fluency is a complex trait that should be discussed, explored, and further developed every year that students learn to write in school; both kindergartners and high school seniors can be taught to think about developmentally appropriate skills that are associated with sentence fluency. This page contains sentence fluency lessons and resources that we consider appropriate for sharing with third graders and up. If you are working with primary writers and the six traits, be sure to visit WritingFix's 6 Traits and Primary Writing Homepage for lessons designed specifically for those grade levels.