When you set up your classroom, it is important to manage the Stream - don’t just go with the flow! The default setting can be a hindrance to learning.
When you create a classroom, go to settings (the small gear icon at the top right) 
Why?
The stream is the first page students see on entering your classroom, so it should not look a mess.
- Allowing students to post would make it messy and distracting.
- If you enable notifications, each time you post something (eg a new lesson), it goes on the stream. The problem is that you might not post work in the order in which you want students to engage with it. You might want to plan and structure work in advance. So teach your students to go to the Classwork page and see the structure and organisation of your course correctly.
- You can still post announcements on the stream that you think are relevant at a particular time. This could be particularly useful in an online teaching context when it would be all the more important to keep the stream uncluttered.
There are some cognitive benefits of having a well-ordered course. It demonstrates that knowledge and ideas are not random, but have an inherent structure.