public void Grade_returns_an_empty_list_if_there_are_no_students_in_that_grade()
    {
        var sut      = new GradeSchool();
        var expected = Array.Empty <string>();

        Assert.Empty(sut.Grade(1));
    }
    public void Grade_returns_the_students_in_that_grade_in_alphabetical_order()
    {
        var sut = new GradeSchool();

        sut.Add("Franklin", 5);
        sut.Add("Bradley", 5);
        sut.Add("Jeff", 1);
        var expected = new[] { "Bradley", "Franklin" };

        Assert.Equal(expected, sut.Grade(5));
    }