I too have wondered if it's possible to change the order that things appear. Right now, it appears to me to be by status (open, closed answers recently available, closed answers available) and then oldest at the top within that status. I would like to have oldest at the bottom instead, so students see more recent assignments at the top.
But I haven't figured out if that's possible to change. What I can suggest to you is not making all of your assignments visible at the beginning of the semester. You'll have to remember to make them visible as you want students to access them. But that would seem to help your problem.
To do that, assuming these sets are already created, go into Homework Sets Editor, select the checkboxes next to all of the sets you want to hide and click on the tab Edit, then Take Action. Then the second column is "Visible" and you can unselect the checkbox from each set you want to hide. (I'm running version 2.9, so your instructions may be slightly different).