The most effective conclusion to Chelsea's essay is:
"As shown, homework is not a necessary factor in student academic success. Students who are given hours of homework each night struggle later in life with finding balance between work and their personal lives. In addition, free time is an important part of the teen years, and students miss out on an important part of their adolescent development due to homework."
This conclusion is effective because it succinctly summarizes the main argument of the essay, reiterates the negative impact of excessive homework on students' lives, and emphasizes the importance of free time for personal development. It maintains a formal tone appropriate for an argumentative essay and reinforces Chelsea's thesis without resorting to overly emotional language or personal anecdotes.