On your school portal, search for "Enrollees List" in the search bar above.
You will then be directed to this page. Select the students you want the grades to be shown to by clicking the checkbox right beside the names. Clicking the uppermost checkbox will select all students currently shown.
After selecting, click "Actions" then "Edit".
In the "Field" section choose "Show Grades in Mobile App", and in the "Value" section, "Yes" or "No", depending on whether you want to show or hide grades.
You can then click "Update" to apply changes.