From what I gather, the instructor relation to class isn't many-to-many for a single course. So should you simply have a table which contains classes related to a single row in InstructorCourses.
In other words a bit like following
Instructor
- Sam
- Tim
Courses
- C#
- Java
Classes
- Class 1
- Class 2
Instructor courses
- Sam, C#
- Tim, C#
- Tim, Java
Instructor course classes
- Sam, C#, Class 1
- Tim, C#, Class 2
- Tim, Java, Class 1
- Tim, Java, Class 2
This is just a data example so of course you should have proper foreign keys pointing to correct tables and so on.