This article explains how to send a class announcement as a broadcast.
Step 1: Click the green "Add" circle and choose "Send Announcement". See below.
Step 2: Click the "Send To" box.
Step 3: Choose "All Students".
Step 4: Once you have selected the "All Students" group write your message in the text box. If you want to attach a document or other media click "Add Attachment" if not, skip to step 6.
Step 5: Choose the type of attachment you want to attach as shown below.
Step 6: Once you are happy to send the broadcast press the green tick in the top right-hand corner of the window. Then confirm this choice by pressing "Send Now". See below.