Is there always an independent clause in every sentence?

Read the rough draft of a student's conclusion to anargumentative editorial.(1) year-round schooling would student achievement.(2) not so much information would have to be crammed intonine months. (3) furthermore, the elimination of long breaks,particularly summer break, would significantly reduce"learning loss" and the additional time wasted at thebeginning of each academic year to reteach previouslylearned material. (4) it is clear that year-round schooling students learn and retain more. (5) the united staleis not the only country to experiment with a year-roundschool schedule. (6) although many still claim that evidenceabout year-round school and its link to higher achievement isinconclusive, these people need to open their eyes and seethat students in these schools consistently score higher oncertain assessments than students who attend schools withtraditional nine-month calendars. (7) it is about time that wereplaced our outdated and inefficient school calendars.which best revises sentence 6 to create a more appropriate and respectful tone