If a student forgets their login details, you can easily retrieve their username and/or reset their password.
Click into the Admin page, and you'll see the list of usernames in the Student list. If a student is not in your class yet, they will need to be added to your class first.
If the student has an email address linked with their account, they can reset their own password by using the prompts at login. Remember that students can log in with either their username or their email address that is linked with their account.
To reset a password for your student, on the Admin page hit the Reset button next to the student's Username. This will generate a generic password, which the student can use to log in. They will then be prompted to create a new password.
See how here: