This feature is applicable to users who are subscribed to:
1) Classroom Lite or Plus
2) Buncee for Schools & Districts
Please watch this video to learn how-to create student accounts:
Note for Buncee for Schools & Districts Users: Creating a class using the aforementioned steps will not create a new class in Google Classroom or Microsoft Office 365. See this article for more details about Roster Sync with Google Classroom and Microsoft Office 365.
For detailed steps on creating classes and student accounts, read this article here.