If the username is correct, but it's still not letting the student log in, the next step would be to change the password. There are two ways to do this: the first way would be through the student search function, either through the search bar at the top of the website, or through the Students tab in the toolbar. The second way would be from the online test roster.
Changing Through the Student Search
Type the student's name into the search bar. Once you choose the student whose password you would like to change, it will take you to the Student Demographics page. At the bottom of the General Information section, you'll find the Portal Password section with a link to Change the password next to it.
Clicking on this will take you to another window that asks for the new password, put in the new, temporary password, and have the student try logging in again.