private void button1_Click(object sender, EventArgs e)
        {
            Teacherlogin teacherlogin = new Teacherlogin();

            this.Hide();
            teacherlogin.Show();
        }
        private void button13_Click(object sender, EventArgs e)
        {
            Teacherlogin TLogin = new Teacherlogin();

            this.Close();
            TLogin.Show();
        }