If you've just created your + Assessment, you should be able to click the applicable column to grade each student.  If you'd like to grade all students with the same term, click Grade All before making your selection.

If you need to go back to grade the assessment, you can find it in Assigned Assessments for the class that you assigned it to.  Hover over the assessment in Assigned Assessments, click the three dots, and click Grade.

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