Right now it just adds the course by default. It's also not intuitive that you have to click the X's to select new courses. You could either:
Have an 'Add Course' button that adds the course to your schedule when you click it and then clears out the course selection fields
Just clear out the course selection fields automatically. This tells the user that the action they were trying to do has been completed. This is what ScheduleBuilder does.
Right now it just adds the course by default. It's also not intuitive that you have to click the X's to select new courses. You could either: