Google Classroom recently added the feature where you can move a post to the top. This kind of allows you to reorganize the stream. In the upper right hand corner of a post notice there are 3 dots to indicate "More options." Click on these 3 dots to access the option to "Move to top."
You can only move one post at a time to the top. If you have 3 assignments #001, #002, and #003 and want them listed in that order you will want to move them to the top in backwards order. First move #003 to the top. Next moving #002 will place this assignment above #003. Then move #001 to the top last. This will cause the assignments to be listed in 1, 2, 3 order.