The easiest way to manage the release of grades in Moodle is via the Gradebook of your course. The "grade" of a given assignment can be hidden before grading, and then all grades released at the appropriate time, by unhiding the grade. If the "Notify student" box is ticked when students are graded, the notification is not sent until the grade is unhidden, and all students will be notified at the same time.
Note: Hiding the assignment drop box itself may not prevent the student from seeing their grade. If a grade has been assigned and has not been hidden in the Gradebook, students may be able to access their grade via their Profile page.
To Hide/Show grades via the Gradebook:
1. From your course, click the Gradebook icon in the blue sidebar, and 2. click the Setup tab
The Setup tab will show whether the grades are hidden, or available to students.
(In the case below, both Assessment 1 and Assessment 2's grades would be visible to students as soon as they are marked.)
To hide the grade for an assignment, click the assignment's Edit link, and select Hide.
The assignment name will now be grey, instead of blue, indicating that the assignment grade is now hidden.
Note: This does not hide the assignment drop box itself.
To release the grade at the appropriate time, click Edit, and select Show.