Google Classroom has a private comment feature for each assignment which allows pupils to ask their teacher a private question about the activity. Pupils should only use the private comment feature if they are unable to ask their teacher a private question about the activity.
The private comment box is on the right-hand side of the assignment pages only and not included on material pages.
On the same page where you can add an attachment is a box that says Add private comment. This is where you can write a message to the teacher about the assignment. Clicking on the paper aeroplane icon sends the message.