Browser refresh problem with ModalPopupExtender

I am using the ModalPopupExtender to add/edit data. It works fine but the problem is that after hiding the modal,if i hit the browser refresh button, the modal appears again (as it did when I clicked on add/edit button). I am using the following code:

<div class="dataentry"><asp:UpdatePanel ID="UpdatePanel1" runat="server" UpdateMode="Conditional"><ContentTemplate><asp:GridView
                                    ID="gvSchool"
                                    runat="server"
                                    DataKeyNames="SchoolID"
                                    AutoGenerateColumns="False"
                                    CellPadding="6" ShowFooter="True"><Columns><asp:TemplateField ShowHeader="True" HeaderText="Edit"<ItemTemplate><asp:ImageButton ID="btnEdit" runat="server" CausesValidation="False" CommandName="EditData"
                                                    ImageUrl="edit.png" ToolTip="Edit record" /></ItemTemplate></asp:TemplateField><asp:BoundField HeaderText="School" DataField="SchoolName" /></Columns></asp:GridView></ContentTemplate></asp:UpdatePanel><asp:Panel
                            ID="pnlAddEdit"
                            runat="server"
                            style="display:none; background-image:url('dialogtitle.jpg');"
                            Height="140px"
                            Width="300px"><span class="paneltitle">School Details</span><asp:UpdatePanel ID="UpdatePanel2" runat="server" UpdateMode="Conditional"><ContentTemplate><asp:Button ID="btnShowPopup" runat="server" Style="display: none" /><asp:ModalPopupExtender
                                        ID="ModalPopupExtender1"
                                        PopupControlID="pnlAddEdit"
                                        BackgroundCssClass="modalBackground"
                                        TargetControlID="btnShowPopup"
                                        runat="server"></asp:ModalPopupExtender><table style="background-color:White; " align="center"><tr><td style="text-align:right;"><spanSchool: </span></td><td style="text-align:left;"><asp:TextBox ID="txtSchool"runat="server"></asp:TextBox><asp:RequiredFieldValidator ID="RequiredFieldValidator1" runat="server" ErrorMessage="*"
                                                    ControlToValidate="txtSchool" /></td></tr><tr><td colspan="2"></td></tr><tr><td style="text-align:right;"></td><td style="text-align:right;"><asp:ImageButton ID="btnSave" ImageUrl="save.jpg" runat="server" Text="Save" OnClick="btnSave_Click"  />&nbsp;<asp:ImageButton ID="btnUpdate" ImageUrl="update.jpg"runat="server" Text="Update" OnClick="btnUpdate_Click"  />&nbsp;<asp:ImageButton ImageUrl="cancel.jpg" ID="btnCancel" runat="server" Text="Cancel" /></td></tr></table></ContentTemplate><Triggers><asp:AsyncPostBackTrigger ControlID="btnSave" EventName="Click" /><asp:AsyncPostBackTrigger ControlID="btnCancel" EventName="Click" /></Triggers></asp:UpdatePanel></asp:Panel></div>

The code-behind code is as follows: 

    protected void btnSave_Click (object sender, EventArgs e)
    {
        try
        {
            // save school result

            // display some user message

            // clear out controls;


            gvSchool.DataSource = // get all schools
            gvSchool.DataBind();
            UpdatePanel1.Update ();

        }
        catch (Exception ex)
        { 
             //display error
        }
    }

I looked at the sample found here: http://www.ajaxcontroltoolkit.com/ModalPopup/ModalPopup.aspx and that does not load the modal popup on browser refresh. Is there a way to hide the modal popup on browser refresh? Am I missing something?

Any help is appreciated.