Setting Field To Null

I have a form to enter attendance in that pulls students from a StudentEnrollmentTable based on FacultyName which is selected from a combobox that runs the following code:

Private Sub cboInstructorName_Click()
Me.Requery
End Sub

A query (StudentAttendanceBYFaculty) is run each time a faculty name is selected. On the form their is a field name TempClassesAttended which is bound to a field of the same name in the StudentEnrollmentTable. Teachers will enter attendance data and run an append query to append the current form records to the StudentAttendanceTable. Each time the form is repopulated the most recent TempClassesAttended values are pulled into the form. This is what is expected.

Now I want to load Null values into the TempAttendance field on the form each time the Faculty selects their name and runs the event. I looked at code online and it seems easy enough, but I don't know enough to make it work. This is the code suggested:

UPDATE TableName SET FieldName = Null
OR
UPDATE MyTable
SET MyField = Null

how to include this into the current event so that the event will return the faculty records with Null values in the TempAttendence field.