To assign a new class to an existing teacher:
Click "add a class"
Create the class name and class code as usual.
If you are the admin, you'll see an option to assign the class to a teacher. Select the teacher fom the drop down menu.
To assign an existing class to a teacher:
Select the class from the green drop down menu.
Click the "Manage Class" button.
Edit the "current teacher" and select the new teacher from the drop down menu.