Hello Felix.  Try watching this video and see if it helps.

https://www.youtube.com/watch?v=zr2gvrIRwew 

For anyone who is still experiencing this issue, please see the following article to ensure that you've taken all of the necessary steps to sync with Google Classroom: https://support.khanacademy.org/hc/en-us/articles/360030760852-How-do-I-import-my-Google-Classroom-roster-to-Khan-Academy-

If you're still having issues, please submit a support ticket and we'd be happy to assist you!