If you're getting an error message saying that your code has already been redeemed, it means that you have already created a username using the code. You don't need to create a new account, and your student code is now invalid. You can now log into your student account at student.classdojo.com with the student username and password you created when you registered for your account.
If you cannot remember your username and/or password, you'll need to do the following:
First: Check your Username
Make sure you are using the correct username. To figure out your username, all you need to do is ask your teacher.
Your teacher will be able to find your username by opening their "Student Connection" page on the website.
Second: Reset Your Password
If your username is correct, have your teacher reset your password via their Student Connections screen. If you have a connected parent, they can also assist you in getting back into your account from their Parent Account Reports screen. Have them login, go to "Your Kids," then click on the "Login Info" link on the right.