Lacking details on what it is that Scheduling gets done, or they don't, its difficult to offer specific guidance.
However, I would say that your auditors are overreaching a bit. ISO 9001/AS9100 require that you determine how the results of your objective are evaluated. I don't read anything in the standard that says yes/no is not allowed as an evaluation criteria, or that an objective has to be growth based. You get to make that decision. In my experience, I've had fewer growth goals, and more specific and constant goal lines that set a threshold for corrective action if they aren't met.
Now back to specifics. If you feel that you have a company benefit to monitoring and improving the scheduling department against a set objective, take a step back and look at what their process is. They get "It" done, or don't. What is "It?" How often does "It" happen? Is there an expectation or requirement that "It" be done within a certain time interval? Are there problems/risks that could take place if "It" doesn't happen effectively? Is there an internal customer that relies on "It?" You might gather the input of the internal customer and find out how Scheduling's output affects them.
None of these are necessarily the right questions, but maybe it might prompt some ideas for you