Well, the best case scenario when a student is below average is that you're going to sell him more training hours, and that he won't pass the test.
If the student does not guarantee a safe flight, he won't have his CPL, whomever pays the FE.
If the student is below average... Well, about half of currently flying airline pilots are below average. That's the definition of average, below average does not mean you're incompetent.
This is all coming from stories heard with several instructors of my th ATPL school.