Assigned students joining a class
If a student is registered to the class, the students are automatically accepted to join the class. A pop-up notification appears to notify the teacher that the student has joined.

 

Not-registered students joining a class
If a student is not registered to the class, they can still join the class. A pop-up notification appears stating that the student wants to join the class. Select the decline or accept button.  Other students cannot ask to join the class until the request is processed.

Go to Toolbar > More Menu > Student to decline a student after accepting them to join the class. Once declined, a student can request to re-join the class.