asp.net c# 删除弹出提示框

<mce:script type="text/javascript"><!--
function Delete() {
var grade = document.getElementById('<%=txtGrade.ClientID %>');
return confirm("要删除"+grade.value+"级的学生吗？");
}

// --></mce:script>
<asp:Button ID="btnDelete"
runat="server" Text="删除数据" onclick="btnDelete_Click" OnClientClick="Delete()"/>

protected void Page_Load(object sender, EventArgs e)
{
btnDelete.Attributes.Add("onclick","return confirm('确定要删除级的学生吗？')");
}