各位高手请教一个问题

我用了模板页，但javascript无法获取内容页中的控件
<%@ Page Language="C#" MasterPageFile="~/Student.master" AutoEventWireup="true" CodeFile="modifPwd.aspx.cs" Inherits="modifPwd" Title="Untitled Page" %><%@ Register Assembly="System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"
    Namespace="System.Web.UI" TagPrefix="asp" %>
<asp:Content ID="Content1" ContentPlaceHolderID="ContentPlaceHolder1" Runat="Server">
    <script type="text/jscript">
        function fnclear()
        {
           document.getElementById('txt_OldPed').value="";
           document.getElementById('txt_NewPwd').value="";
           document.getElementById('txt_RePwd').value="";
        }
    </script>
       <asp:TextBox ID="txt_OldPed" runat="server" Width="129px" TextMode="Password"></asp:TextBox>
       <asp:TextBox ID="txt_NewPwd" runat="server" Width="129px" TextMode="Password"></asp:TextBox>
       <asp:TextBox ID="txt_RePwd" runat="server" Width="129px" TextMode="Password"></asp:TextBox>
        <input id="btn_clear" style="width: 65px" type="button" value="重新填写" onclick="fnclear();"    runat="server" />