Scroll down within the pop-up to hit the Google Classroom Sync button. This will bring in your classes and students created within Google Classroom.
After the sync is successful, notice (upon page refresh) that your number of active students on the dashboard has gone up. You can click into the number of students to see which students were brought in from the sync.
Assigning a bulb to your Google classroom is as easy as pasting in your share url into the Google Classroom you are assigning. More info here.
*note if your student is having trouble with the Google Classroom sync, we recommend having them turn off any pop-up blockers