You can create an "Archive" or "Favorites" folder inside each one of your Google Classrooms and copy or move the files that you used this past year inside those folders. That way, you still have all the files that you used for that particular classroom that you can go back and look at when you start your classes for the coming school year. It would be good idea to put the year of the newly created folder so you can keep track of what year you used particular assignments, files and links.
If you are basically teaching the same classes next year, you don't have to delete the entire class and start all over. You can simply delete the students from this year. However, remember that Google Classroom stacks assignments and announcements by the date you entered them so what you want for the beginning of the year will be at the bottom.
What is my plan for my Google Classroom next year? I plan to archive my classes so that I still have access to my files and links. However, I plan to recreate all new Google Classrooms for next year.
From the home page, you can "Archive" the class.
Archiving does save the class, but it removes the class from the home screen. It also removes the class from the Classroom menu and the ability to add to the class or edit by teachers or students is disabled. Archived classes appear in the drop down by the HOME menu.