Create a Forum as specified here. At the bottom of the page for Add a Forum, there is a tab to 'Restrict Access'. Click on this option to open up more options.
Clicking on 'Restrict Access' will open up this dialogue box. Click on User Profile. This will allow you to restrict access to a single student.
Clicking on 'User Profile' will lead you to this set of options. Make sure that 'Email address' is selected under 'User Profile Field' and that the second field is 'contains', NOT 'equal to'. In the third space, you may enter the student's email username and click 'Save and return to course'.
Please note that this method requires that you make a Forum for each student.
Another extremely helpful document to consult when setting up a forum using groupings is Using Groups with Moodle Forums. It offers a clear explanation of how the Common module setting options of "Separate Groups" vs "Visible Groups" affect the use of Moodle forums so you can choose the option that will work best for your purposes.