By default, new topics and/or assignments will go to the bottom of the list on the Classwork tab. You can move topics and assignments within topics up and down on the Classwork tab by clicking the three dots to the right and selecting "move up" or "move down".
However, if you have a long list of previous assignments, this is a lot of moving up/down after adding a new assignment. There is no way to archive or hide previous assignments, other than deleting them. However, I don't recommend deleting past assignments as you won't be able to "reuse" them in later Classrooms. Additionally, once an assignment is deleted, the grades and any private comments are also deleted.
If you create an assignment, question or material without assigning it to a topic, it will appear at the top of the Classwork tab.
Find the topic dropdown and add a topic to the post by creating a new topic or selecting an existing topic. This will move the assignment from the top of the stream into a topic module, clearing the way at the top of the Classwork tab for the next set of new assignments.
In my opinion, this is the easiest way to get new assignments to the top of the Classwork tab without having to endlessly click "move up" or "move down". Let me know how this works for you in the comments!