public void T5_adding_teacher_to_student()
        {
            IStudent s = SchoolFactory.CreateStudent("Thibault Cam", 9, DateTime.Parse("21/02/1994"), "IL");
            ITeacher t = SchoolFactory.CreateTeacher("Olivier Spineli", "C#", "IL", DateTime.Parse("21/02/1994"), true);

            Action action = (() => s.AddTeacher(null));

            action.Should().Throw <ArgumentNullException>();

            s.AddTeacher(t);
            s.MainTeacher.Should().NotBeNull();
            s.MainTeacher.Should().BeEquivalentTo(t);
        }