遍历Repeater批量更新数据，为什么只修改了第一行，新手，求指导

遍历Repeater批量更新数据，为何只修改了第一行，新手，求指导
前台

<asp:UpdatePanel ID="UpdatePanel1" runat="server">

                <ContentTemplate>                

                <asp:CheckBox ID="ckbAll" runat="server" Text="全选/反选" Font-Size="Larger" 

                    oncheckedchanged="ckbAll_CheckedChanged" AutoPostBack="True" />

                    <asp:Label ID="lblTishi" runat="server" Font-Size="Larger" ForeColor="Red"></asp:Label>

                <table style="width:97%;" class="tablelist">

                    <asp:Repeater ID="rptStudent" runat="server">

                    <HeaderTemplate>

                        <tr>

                        <td class="HeadTea_sure" width="8%">

                            选择</td>

                        <td class="HeadTea_sure" width="8%">

                            姓名</td>

                        <td class="HeadTea_sure" width="8%">

                            性别</td>

                        <td class="HeadTea_sure" width="25%">

                            身份证号</td>

                        <td class="HeadTea_sure" width="25%">

                            地址</td>

                        <td class="HeadTea_sure" width="12%">

                            电话</td>

                        <td class="HeadTea_sure" width="10%">

                            是否审核</td>

                        

                        </tr>

                    </HeaderTemplate>

                    <ItemTemplate>

                        <tr>

                        <td class="HeadTea_font" width="8%">

                            <asp:CheckBox ID="ckbStu"  runat="server" Text='选择' CssClass='<%#Eval("UserId")%>' />

                            </td> 

                        <td class="HeadTea_font" width="8%">

                           <%#Eval("StudentName")%> </td>