1. Log onto your account as a teacher
  2. Go to the course page
  3. Go to the class page
  4. Select the Students tab
  5. You can send a message to a single student by clicking on the Envelope under his/her picture
  6. You can send a message to a parent by clicking on the Contact Parent icon under the student's picture
  7. You can select multiple students and send them a message by clicking on the "Send Message" button
  8. You can select multiple students and send a message to their parents by clicking on the "Contact Parents" button 
  9. Now type in the message subject and body
  10. Press send message

Have a look on the following video :