Sharing assignments to Google Classroom is possible if you chose to pair a folder to a Google Classroom class when the folder was created.
To share assignments to a paired Google Classroom class, or later remove them, navigate to the folder containing the assignments you wish to share, check the required assignments, and click the "Share to Google Classroom" button:
In the pop-up window, choose whether to share or remove the selected assignments:
After sharing, the specified assignments will be available in the paired Google Classroom class stream: