In case you have published quiz grades but would like to retract them, please follow the steps below. When retracted, grades are no longer visible to students.
1. Click on Quizzes in the navigation bar under course tools, then select the quiz you want to change. Click on the drop down menu next to the quiz, and select Grade.
2. Multiselect the users for the quiz and click on Retract Feedback.
3. This option will automatically retract the grades from the grade book only if Allow automatic export to grades is enabled in the Assessment tab.