This means your school has signed up for school or district features on Edmodo. You'll need to login to Edmodo from your school or district login page which looks something like yourdistrictname.edmodo.com. You will need to get the code from your school admin. This code verifies that you are a teacher at the school or district.
Note: If you did not create your account using a school subdomain, there is no need to delete your account and create a new one. You can use your existing account. You will only need the school code to log into your school subdomain.