protected void Login_Click(object sender, EventArgs e)
        {
            da = new DA.DA();

            string username = txtUsername.Value;

            string Login = da.CheckUSer(username, txtPassword.Value);

            if (Login == "1")
            {
                //  1 = Teacher
                Session["username"] = username;
                lblError.InnerHtml  = "";
                Response.Redirect("ClassView.aspx");
            }
            else if (Login == "2")
            {
                // 2 = Student
                Session["username"] = username;
                lblError.InnerHtml  = "";
                Response.Redirect("StudentPage.aspx");
            }
            else if (Login == "3")
            {
                // 3 = Admin
                Session["username"] = username;
                lblError.InnerHtml  = "";
                Response.Redirect("AdminPage.aspx");
            }
            else
            {
                lblError.InnerHtml  = Login;
                Session["username"] = null;
            }
        }