From your courses page, select the option to manage your school's courses.
From there, you can see all the active and draft courses for your school. For any active course, you can use the actions dropdown menu to assign the course to classes.
This option is best for assigning a single course to multiple classes.
You can also assign courses to your classes directly from the class list. From the menu, navigate to the Classes & Settings page.
Select the class you want to assign a course to.
At the top of the page, it will show any assigned courses for the class.
Click on the button under Assigned Courses and a menu will appear that will allow you to select from any of your school's active courses.