The most effective claim for Cleo's argument is:
Kennedy High School should open at 8:30 a.m. instead of 7:30 because students would get an extra hour of sleep, improving their health and brain function.
This claim is strong because it directly addresses the benefits of additional sleep, which is a widely recognized factor in enhancing students' overall well-being and academic performance. It provides a clear rationale for the proposed change and focuses on a positive outcome that can be supported by research on sleep and health.