You have two options to make an announcement:
A new post to a discussion forum with students subscribed so they receive an email with each new forum post. Be sure to set Force everyone to be subscribed in the forum settings (Yes, forever or Yes, initially). Otherwise, students will not receive an email. It is also recommended that you turn on Read tracking for this forum to flag new posts as new.
Option 2: Add an announcement in the top content area
Step 1: Turn editing on and click on the Add a resource drop-down menu inside the first topic area. Then select Insert a label.
Step 2: Type what you wish to announce in the text box. Choose the size, font and style desired.
Step 3: Back on the main page, this is what the announcement looks like: