How can I manually add data to a dataGridView?

There are 0 cells in the newly created row, that's why you are getting that exception. You cannot use statements like

row.Cells[0].Value = i.ToString();

unless you manually add cells to the blank row.

My version of this:

                OracleCommand cmd = new OracleCommand(commandText, _oraConn);
                OracleDataReader dr = cmd.ExecuteReader();

                int i = 0;
                while (dr.Read())
                {
                    DataGridViewRow row = new DataGridViewRow();
                    row.CreateCells(dataGridView1);


                    row.Cells[0].Value = dr.GetValue(0).ToString();
                    row.Cells[1].Value = dr.GetValue(1).ToString();
                    row.Cells[2].Value = dr.GetValue(2).ToString();
                    row.Cells[3].Value = dr.GetValue(3).ToString();
                    row.Cells[4].Value = dr.GetValue(4).ToString();
                    row.Cells[5].Value = dr.GetValue(5).ToString();

                    dataGridView1.Rows.Add(row);

                    //MessageBox.Show( dr.GetValue("vz_id").ToString());
                    i++;
                };

Thanks for your answers.

Its simple,

myDataGridView.Rows.Add(value1, value2, value3...);

It worked when I had configured my DGV previously for the coming data columns through the GUI. So in your case, it would be like:

private void LoadStudentGrades(int gradeParaleloId, int subjectId)
{
    GradeStudentRepository gradeStudentRepo = new GradeStudentRepository();
    students = gradeStudentRepo.FindAllGradeStudents().Where(g => g.GradeParaleloId == gradeParaleloId).Select(g => g.Student);

    int i = 1;
    foreach (var student in students)
    {
        dataGridView1.Rows.Add(i.ToString(), student.LastNameFather + " " + student.LastNameMother + ", " + student.Name);
        i++;
    }
}

May be you have to configure the DGV for the columns and their names separately.

You are just missing one line :-P

DataGridViewRow row = new DataGridViewRow();
row.CreateCells(dataGridView1);  // this line was missing
row.Cells[0].Value = "Cell1";
row.Cells[1].Value = "Cell2";
dataGridView1.Rows.Add(row);