The piece of evidence that best supports Sarah's claim that students should have less homework is: "Students are staying up late completing homework and not getting enough sleep."
This evidence is strong because it shows a negative impact of too much homework on students' health. If students are losing sleep because they have too much homework, it can affect their ability to learn and be successful in school. This helps explain why having less homework could be better for students, as it would allow them to get the sleep they need to do well in their studies.