Go to Inbox
Create New Message
Click on the pencil/page icon
Select the Course
If you select the course at the top then when you go to recipients all you will see are those from within the selected course.
If no course is selected
If you don't select a course then when you click on the TO field you'll get a list of classes
The first line is ALL IN STUDENTS. If you choose this it will send the message to ALL the students. You should see all their names in the TO field. I have noticed that it is relatively easy to hit that.