Click Reuse Post to select content from any of your current or archived classes, and post them to a new or existing class (see animated GIF).
If you found the groove in Google Classroom (Topics are set up in a way that worked with students, for example, and more of the Posts are workable) you can Copy the Class with a new name for next year.
When you are done combing through and/or copying content from old classes, clean up your Class list by Archiving them. You will still be able to see old work, student interactions, and lessons learned by clicking the 'Archived Classes' link at the bottom of your class list.