You can find this in your Class Settings on the main dashboard 

Once you have clicked class settings , a pop up will appear and you can check the box Close Registration

The second way to close registration is in your class dashboard press (+) Add Students 

This will bring a pop up to the screen where you can choose how to add your students

If  you have chosen to have students create their own accounts a pop up box will appear and allow you to check the close registration box.

Once you've checked the box you will get a notification saying that your registration is closed .

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