Google Classroom grade passback is a Silver/Gold plan feature that allows teachers to return scores from Formative assignments back to Google Classroom. 

If your account is part of a Silver/Gold plan, click on the triple dots at the top right of your formative page, and select "Grade Passback":
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Then "Pass back grades to Google."

You will need to select your Google account from a window, and then the Google Classroom Grade Passback form will pop up:

This form will list all the Google Classroom classes and students that have been imported into Formative. You can choose to pass back grades for all or a selected number of classes and/or students. 

The initial "Max Points" value is the points total of all the items in the Formative. Note: Google Classroom point values MUST be whole numbers.

By default, Formative will set the grades in Google Classroom as "draft". This means only the teacher can view the grade. But if you select the option to "Show Scores to Students," then the grade will become "assigned" in Google Classroom, and will become visible on each student's dashboard:
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Note that only students who have submitted the work in Google Classroom can receive an assigned grade. If not submitted there, the grade will default to a draft grade. The image below shows what a teacher sees when they select Pass Back Grades to Google for a formative. Student 5 has completed the formative and clicked "Mark as Done" in Google Classroom. Student 6 completed the formative but forgot to click "Mark as Done". The other students have not yet completed the formative at all.

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Once the form is submitted, you will be updated with the status of the grades:

If this warning appears under a class in the form, then you did not choose the "Assign & Post to Google" button when you assigned the formative. You must choose this option in order to pass the grades back to Classroom. If you did not see this option when you assigned, then the class you chose was not a class you imported from Google Classroom.

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You may see this message if you updated the assignment after students had taken it. Go back to "Assign," and update the settings to include all the students who have taken it, then click "Update & Post to Google." You will now be able to pass the grades back for all students.

You may see an "Oops! Something Went Wrong!" message saying "Course work max points is invalid." Google Classroom has changed their system and no longer supports grade pass back for partial points.