You must hide the individual assignment grade item while you are marking to ensure that marks and feedback are released at the same time to all students. To do this:
From the Gradebook setup tab you can also hide the Module total. This is necessary if students can work out their hidden grades from the total or if you have not set up your weighting in the Grader Report and the Course total is incorrect.
A module can have more than one grade category (e.g. Formative and Summative). You can hide all the grades within a category by hiding the grade category.
Grade categories are displayed in a folder icon and highlighted in grey.
When you are marking student assignments and have hidden the assignment via Setup in Grades, do not make any changes to the setting Notify students in the Assignment settings or within the assignment grading screen. If you change this to No at any stage in the marking process an automatic notification from Moodle will not be sent to students informing them of the release of their grades and feedback.