updatepanel没有实现局部刷新

<asp:UpdatePanel ID="UpdatePanel1"
                                        runat="server" UpdateMode="Conditional">
                                        <ContentTemplate>
                                            <asp:DropDownList ID="DropDownList1" runat="server" AutoPostBack="true" OnSelectedIndexChanged="DropDownList1_SelectedIndexChanged">
                                                <asp:ListItem Text="请跟进" Value="请跟进"></asp:ListItem>
                                                <asp:ListItem Text="请核实" Value="请核实"></asp:ListItem>
                                                <asp:ListItem Text="已报名" Value="已报名"></asp:ListItem>
                                                <asp:ListItem Text="未报名" Value="未报名"></asp:ListItem>
                                            </asp:DropDownList>
                                            <asp:HiddenField ID="HiddenField1" runat="server" Value='<%#Eval("id") %>' />
                                        </ContentTemplate>
                                    </asp:UpdatePanel>

protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
        {
            //string value = ((DropDownList)dataListSchool.FindControl("DropDownList1")).SelectedItem.Text;
            foreach (DataListItem item in dataListSchool.Items)
            {
                if ((item.ItemType == ListItemType.Item ) || (item.ItemType == ListItemType.AlternatingItem) || (item.ItemType == ListItemType.SelectedItem))
                {
                    string value = ((DropDownList)item.FindControl("DropDownList1")).SelectedValue;
                    int id = Convert.ToInt32(((HiddenField)item.FindControl("HiddenField1")).Value);
                    switch (value)
                    {
                        case "请跟进":
                            DbHelperSQL.ExecuteSql("update person set mark = '请跟进' where id = " + id);
                            break;
                        case "请核实":
                            DbHelperSQL.ExecuteSql("update person set mark = '请核实' where id = " + id);
                            break;
                        case "未报名":
                            DbHelperSQL.ExecuteSql("update person set mark = '未报名' where id = " + id);
                            break;
                        case "已报名":
                            DbHelperSQL.ExecuteSql("update person set mark = '已报名' where id = " + id);
                            break;
                        default:
                            break;
                    }
                }
            }

        }