If your password is not working you can check with an account administrator to make sure your credentials have not been changed. If your account is connected to LDAP, you will need to make sure you are using your network username/password combination.
Student users may need to contact their account administrator in order to reset their password.
Provided your account administrator allows the "Act As" feature and also allows students to change passwords, a teacher may "Act As" the student and log in as the student to reset the password to something temporary. The student can then log in using the temporary password and change it.
The password change is performed on the My Profile -> Change Password section of My Settings.