Leslie doesn't fit in and gets bullied a lot by his classmates, so Jess gets frustrated with her. Because of his feelings about Leslie, he couldn't sleep because of worrying about it too much.
students'
This is because in, "We were impressed by several students' drawings," many students are owning many drawings.
student's will not work because this sentence does not say that only one student owns this one drawing.
students will not work because this is not possessive.
I had this question once, the answer for mine was a "reason" as to the world problems. Only because some people believed it had a catastrophic impact on the nations
