If you need to create a new password for one of your students all you need to do is:
Log into your admin account
Go to the users page and search for the student using the search boxes
Select the student from the results. Watch out! If there is more than one result ensure you select the correct account. You can check the 'tag' or 'permission' column to verify this.
Then click on the blue generate new password button
A temporary password will be displayed for you to pass on to the student. The first time they use this to log in they will be prompted to set their password to something memorable.