To open the Manage Class menu from the teacher dashboard, click on the gear in the upper left hand corner, above the student list.
A modal will appear with the name of the student, their email address associated with their Desmos account, and an option to hide the student from the dashboard. If students don't have an email linked to their account, it means that they have not yet signed in and won't be able to access their work after they leave the activity.
To hide students, simply check the box in the row of the student you'd like to hide. You can always unhide the student by unchecking the box.
Click the "x" on the modal to go back to the teacher dashboard. All hidden students will no longer be shown in the student list on the teacher dashboard and their answers will be removed from view.