Also, each student should only take (pay for) each course once.  So I would set both CustomerID and CourseID in the junction table as PK:

CustomerID   PK
CourseID   PK
DateEnrolled
CertificationExpires

Or you could create an unique index of CustomerID + CourseID in the junction table.

I ran into these situations often in my old job.  Rather than assigning an arbitrary PK for the junction table, I would often check to see if existing fields should be used as PK instead.

Valid points. Lots of ways you can handle that, but you also might need to allow for the possibility of students retaking courses. In that case, you'd need to allow the same customer to sign up for the same course more than once. Again, though, now we're talking business rules, not database rules.