Anonymous users will be blocked from Google Meet video conferences held by schools, colleges, and universities in a bid to stop pranksters disrupting lessons.
The FBI's Boston field office issued the warning after hijackers infiltrated two school-held Zoom sessions to harass a teacher and display the swastika sign.
Coronavirus means people across Europe are self-isolating and working from home, but users of Microsoft's Slack competitor struggled to log in and send messages to colleagues.