Many-to-many unidirectional, join table empty after persist

Hi,
I have got a many-to-many relationship between entities instructor and student. The student is saved and persisted first. Then I create a new instructor and try to add the student to the studentList in instructor. Then I save (persist) the instructor. The instructor is persisted but the jointable is empty. Any ideas?

@Entity
@Table(name = "instructor")
@PersistenceContext(unitName = "processEntities")
public class Instructor extends Person {

  private static final long serialVersionUID = 1L;

  @ManyToMany
  @JoinTable(
        name = "instructor_student", joinColumns = @JoinColumn(name = "instructor_id"), 
        inverseJoinColumns = @JoinColumn(name = "student_id")
        )
  private List<Student> students = new ArrayList<Student>();

 public List<Student> getStudents() {
    return students;
 }

 public void setStudents(List<Student> students) {
    this.students = students;
 }  
}

ThesisFirstInstructor.java

@Entity
@Table(name = "thesisfirstinstructor")
@PersistenceContext(unitName = "processEntities")
public class ThesisFirstInstructor extends Instructor {}

Student.java

@Entity
@Table(name = "student")
@PersistenceContext(unitName = "processEntities")
public class Student extends Person {

  private static final long serialVersionUID = 1L;

 private String matrNo;

 public String getMatrNo() {
    return matrNo;
 }

 public void setMatrNo(String matrNo) {
    this.matrNo = matrNo;
 }

BusinessLogic.java (EJB)

{
...
Student currentStudent= studentDAO.findByThmUid(currentStudentThmUid);


ThesisFirstInstructor thesisFirstInstructor = new ThesisFirstInstructor();
thesisFirstInstructor.setFirstName(thesisFirstInstructorCamundaUser.getFirstName());
thesisFirstInstructor.setLastName(thesisFirstInstructorCamundaUser.getLastName());
thesisFirstInstructor.setThmUid(thesisFirstInstructorThmUid);

thesisFirstInstructor.getStudents().add(currentStudent);
        
thesisFirstInstructorDAO.persist(thesisFirstInstructor);

}

Unfortunately, the join table is empty![grafik|261x170]:

grafik

Entity thesisFirstInstructor is persisted.
Any ideas?