@ckane
I am not sure if I completely understand what you are trying to explain, but one thing I thought of is the following(may not be desirable, but it would work)
Create A module for each Trimester. In that module place all the assignments for that trimester.
When the assignments are no longer to be visible, unpublish the module.
This will work provided that you also hide the assignments navigation link - which you may not want to do. With the assignments navigation link removed, all assignments would have to be placed in their own module.
Edit: Just reread your post. I don't think the above would hide the information from their to-do list or Calendar, so they would still see the assignments that way.
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