You can use Conversations to send a message to your entire class. Depending on how permissions are set, you may or may not have the ability to message the entire class.
Compose Message
Click the Compose icon to start a new message.
Select Course
Select the course you wish to message from the course drop-down menu.
Open Address Book
Click the address book to send a message to the entire class.
Select Class
Click the name of the course.
Add Message
Add a subject line and type in the message field. Click the Send button when you are finished.