To post an announcement:
Click New Announcement at the top of the Stream.
Type your message.
You can add attachments:
Google Drive: Attach files directly from your Drive.
YouTube: Embed an educational YouTube video.
File: Upload a file from your computer.
Link: Share a website link.
You can choose which classes and even specific students (if desired) receive the announcement.
Click Post to publish it immediately, or click the dropdown arrow next to Post to Schedule it for a future date/time or Save draft.
You can control what appears on the Stream by clicking the Gear icon in the top right corner of your class page. Under "General" you'll find "Stream":
Students can post and comment: Students can start new posts and comment on existing ones.
Students can only comment: Students can only comment on teacher posts.
Only teachers can post or comment: Locks down the Stream for teacher-only communication.
Click the Settings (Gear Icon) in the top right.
Scroll down to the General section.
Find Classwork on the stream.
Change the setting to "Hide notifications."
Now, the only things that will appear on your Stream are the custom announcements and directions you write yourself.