private static Diagram createInitialModel() {
       Diagram diagram = SchoolFactory.eINSTANCE.createDiagram();
      // note that you need to pay attention to the hierarchy that you execute the commands. For example, if you wanted to add students too, you would have first to append the students into the classroom list and then add this classroom to the school.
       School school = SchoolFactory.eINSTANCE.createSchool();
      Classroom room1 = SchoolFactory.eINSTANCE.createClassroom(); 
      Classroom room2 = SchoolFactory.eINSTANCE.createClassroom();
      school.addClassroom(room1); //or whatever name is used for the method that adds a classroom to the school classroom list
      school.addClassroom(room2);
      diagram.setSchool(school);

     return diagram;
}