Teachers have the ability to edit student accounts, and change their passwords. If a student forgets his or her password, you can change it to something that they will remember by finding the student's name in your classroom, clicking on the student's name, and then clicking 'edit,' in the 'author's tools' bar, directly under their username.
You will have to write the password in twice, once in each box, to confirm it. You will be given a rating of high, medium, or low for your password's security level. Even if it reads 'low' you can still use that password.
Scroll down to the bottom and click save. That student should now be able to log in with their new password.