Hiding/encrypting a querystring variable

i have a templatefield and  will like to hide the value of the student variable
when passing this to the next page. I decided to use the httputility.urlencode
to make this value non meaningful but this is not working for me. I am still
getting the same value. How can i do this?

asp:TemplateField HeaderText="picture">            
                <ItemTemplate>  
                  <asp:HyperLink ID="Label1" runat="server" Text='<%# Bind("picture") %>' NavigateUrl=<%#"javascript:my_window=window.open('displayImage.aspx?student=" + UrlFullEncode(DataBinder.Eval(Container.DataItem,"student").ToString()) + "','my_window','width=300,height=300');my_window.focus()" %>/>
                </ItemTemplate>
            </asp:TemplateField>


private const string _strApostropheEncoding = "%27";
    public static string UrlFullEncode(string strUrl)
    {
        if (strUrl == null)
            return "";
        strUrl = HttpUtility.UrlEncode(strUrl);
        return strUrl.Replace("'", _strApostropheEncoding);
    }