Google's QR code log in method 'experiment' for accessing accounts on public computers appears to have run its course.

To log in to Google Classroom, begin by opening a web browser and navigate to classroom.google.com Next, enter a personal, organisation, or educational email account and proceed by pressing Next.

How to join a Google Classroom meeting from a link or code on any device How to log in to Google Classroom and access your course materials on a computer or mobile device ...