I placed a tag with each of the hours on the end of a sheet of tag board. When a class has placed their work on the rack, I place my tag board on the rack to separate the hours. Now that good... but it gets better.
When I take the papers off the rack and put them in a pile, I place the marked tag board on top of the pile to help me stay organized.
PS. A second way that I separate the piles is by switching the color of the paper that I have the kids mount their work on.