To add or remove a teacher from a class, go to the class Admin page and click the green Edit button.
To add a teacher, click in the Teachers box, begin typing the teacher's name (or scroll through the options in the drop-down menu) and click the teacher you'd like to add. Click on the green Save button, and that teacher will now have access to the class.
To remove a teacher, click the dark grey X next to their name in the Teachers box. Click on the green Save button, and that teacher will no longer have access to the class.