This Add-on will load a sidebar menu for you and automatically populate your Google Classrooms into a drop down menu. You can then select the class you would like to import grades from. Once done you will receive a list of all of the assignments in another drop down menu. Select the assignment you want to import and the data will to transferred from Google Classroom to your Google Sheet. You can import student name details as well as the assignment.
You can also Export from your Google Sheet to Google Classroom. This will create a new assignment on Google Classroom and populate the marks from your Google Sheet into the assignment on classroom.