Answer :
In the sentence : I cold not find my homework between the papers in my messy locker .
It states that the locker is messy, from that we understand that there are a lot of papers not only two. So we can say "among" should be used not "between".
It states that the locker is messy, from that we understand that there are a lot of papers not only two. So we can say "among" should be used not "between".