These instructions demonstrate how to turn off student commenting in Google Classroom. Please note: you will need to do this separately for every Classroom.
Step 1
From the Google Classroom main screen, click on the name of the class you want to edit.
Step 2
Click on the gear icon on the top right of Google Classroom to get to the classroom settings. Scroll down to the "Stream" option under General.
Click on the dropdown where it says "Students can post and comment" and change it to the preferred setting. If you choose "Students can only comment", they will be able to comment on anything you or another teacher posts, but they will not be able to initiate a post. If you select "Only teachers can post or comment", the students will not be able to post or write a comment on anything in the stream.
You can always come back and change this setting in the future.