We had a Supreme Court ruling in 2012 that said with the copies that teachers make on behalf of their students, the purpose to be considered is the purpose of the student. It's not the teacher who needs the copies, it's the student.
When you talk about fair dealing and that it's okay for a student to make a copy, those copies being made by instructors are for the benefit of their students. I believe that fair dealing allows those copies to be made on behalf of students.