ComboBox.SelectedValueChanged + SQLConnection

private void LoadSchoolchildsInGpoup()
{
    dataGridViewFastenSchoolchildsInGpoup.Rows.Clear();
    DataGridViewComboBoxCell Cell;
    using (SqlConnection connection = new SqlConnection(connectionString))
    {
        connection.Open();
        queryString = "SELECT ID_Schoolchild, Family, Name, Patronymic, Birthday, EnrollmentDay FROM Schoolchilds WHERE ID_Group = '" + comboBoxFastenSchoolchildsInGroup.SelectedValue + "'";
        SqlCommand command = new SqlCommand(queryString, connection);
        SqlDataReader reader = command.ExecuteReader();
        ushort i = 0;
        try
        {
            while (reader.Read())
            {
                String[] row = { reader[0].ToString(), reader[0].ToString(), reader[1].ToString(), reader[2].ToString(), reader[3].ToString(), reader[4].ToString(), reader[5].ToString() };
                dataGridViewFastenSchoolchildsInGpoup.Rows.Add(row);
                Cell = (DataGridViewComboBoxCell)dataGridViewFastenSchoolchildsInGpoup.Rows[i++].Cells[1];
                Cell.Value = reader[0];
            }
        }
        finally
        {
            reader.Close();
        }
        connection.Close();
    }
}

private void comboBoxFastenSchoolchidsInGroup_SelectedValueChanged(object sender, EventArgs e)
{
    LoadSchoolchildsInGpoup();
}

connection.Open();

LoadSchoolchildsInGpoup();