Enter students into new records created on a layout based on the Student Table. Enter courses into new records created on the Course table.
Use a portal to Join placed on a student layout, to link that student to existing courses records. Use a portal to the join table on a courses layout to link students to the current course.
student::StudentID = Join::StudentID //enable "Allow creation of records via this relationship" for Join here
Course::CourseID = Join::CourseID //enable "allow creation of records via this relationship" for Join here.
In a portal on the student layout, add a drop down or pop up menu for selecting the course. In a portal on the course layout add a drop down or pop up for selecting a student.
Here's a demo file that matches contracts to companies in a many to many relationship: http://www.4shared.com/file/PLhjErzu/Contracts_to_Companies.html
Many thanks for the file and the explanation. That will get me out of my mental logjam....