You can export a lesson's marks as a CSV, for your own mark book or to upload into your school's system.
Downloading them
Open the classroom from the sidebar, go to Assignments, click the assignment and then the lesson.
On the Students tab, tick the students you want in the export, then click CSV above the table. The button stays greyed out until at least one student is ticked.
The download only includes the students you've ticked. To export everyone, tick the checkbox at the top of the table. If you've searched or filtered the list first, that selects everyone who matches, so the download matches what you can see.