One of my favorite add-ons and extensions is Doctopus and Goobric. With Doctopus and Goobric, you import your classes into a Google Spreadsheet, create a rubric, and grade these assignments with the rubric attached. It is an amazing and useful tool for the classroom.
In the presentation below, I have attached directions and videos to use Doctopus and Goobric in your own classroom.
If you have any questions, please let me know!