If you have an existing account, go to Settings and click "progress sharing" for the space to enter a classroom code. Enter the code above.
If you are new to Duolingo, you can download the free app on your phone or use the website linked below to create an account. If you are asked for a classroom code in the beginning, enter the code above. Otherwise, once your account is created, go to Settings and click "progress sharing" at the bottom and enter the code above.
The first extra-credit assignment is available through March 10. Completing it will add five points to your oral presentation grade.