Class Code
You can check the "Class Code" from the "Student List" in the Class Management Tool Index.
By having students enter the Class Code, you can add students to the class afterwards, even if unregistered students attend the lesson.
Note: Teachers cannot join a lesson using the Class Code. Please join a desired lesson from "Add Subject".
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▼How to Enter Class Code
https://gyazo.com/594ebfd7f5a063dfc767b37e644776bf
Reference
Resetting the Class Code
Adding Students to an Existing Class from Admin Console
How to unregister a student participating in a class