There are different thoughts and ideas around Google Classroom and new grading periods. The choices tend to be to keep the same Classroom all year, start a new one every grading period, duplicate your current classroom and delete the assignments and lastly create grading categories for each grading period so that grades can be zeroed out from the gradebook. Please check out the resources below to see which best fits your needs.
Creating a New Classroom Each Grading Period
Marking Grading Periods in Grade Categories
Creating new Classroom, Achieving old Classroom and deleting Assignments
Video with several ideas, including Adding Grading Period Topics
Quick way of adding your students to your new Classroom
Tutorial on how to invite students to your new Classroom and reusing posts
Alice Keeler on moving students to a new Classroom