To message parents/guardians you would go to the Messages section on the bottom > Click on the pencil and paper in the top right hand corner to initiate a message > Type in the name or filter for who you would like to message > Click Create > Type in your message at the bottom > Click Send when you are done. 

If you would like to respond to a message that was sent to you, click on the message > Type in your response > Click Send when you are done.