Suggestion: Saving Students Usernames/Passwords
Create a Google Form and have students fill it out with their usernames and passwords. You will receive all of their usernames and passwords in a spreadsheet. This will prevent students from having to create a new account if they forget their account information.
All students at our school have GMail accounts. If they are logged in to their account, they can log in to Duolingo using the dropdown "log in with Google+" that way they don't have to remember 2 log in names and passwords.