Entering grades into a gradebook should be easy. Moodle helps you by allowing the user to sort students by name four different ways.
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After clicking on "Grades" in the administration menu you will be taken to the "Grader Report". Here you need to focus your attention on the header, "First Name/ Last Name"
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Clicking on "First Name" will sort your students by their first name in alphabetical order.
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Click "First name" again and students will be sorted in reverse alphabetical order.