Below is an example of why one should not trust a high-school diploma to indicate mastery of much of anything.
Two weeks ago, I was approached by the school social worker to be party to a deal made for a student in danger of failing Algebra 2. A senior, this student was in real danger of not graduating in large part because he skipped much of the second and third quarter. The deal went like this: the student would promise to attend every single Algebra 2 class, attend every single one of my Study Skills classes (where he could recieve tutoring), and once these criteria were filled, the student would be allowed the chance to pass the class. I entered into this agreement with the stipulation that, as this student has had several allowances made for him, this would be his last chance. Miss a class and the deal is void.
The next day, he failed to attend my Study Skills class. Thus, per our agreement, the deal was void. Or so I thought.
Yesterday, when I was off because I had just gotten married, the social worker did what she promised she wouldn't do: she reinstated the deal. When I asked her and the vice principal why, they both told me - this only seems like a tall tale - that we just wanted to get the student through, and it would affect our numbers if we did not. Besides, they said, giving him the piece of paper might allow him to get a job that he couldn't get without the diploma.
So, you heard it here first (or maybe you are already familiar): diplomas often mean little more than that teachers and principals let a student skirt by so as to boost their numbers and pass the buck.
These are the days when I really question why we have diplomas in the first place. If they are not standards based, or if standards are so movable that they are merely inconveniences to be stretched, I am not sure what diplomas are to signify. Surely not academic mastery! That wouuld...gasp...expect something out of students and hold them to expectations. (How dare we!)
Thus, it may be in employers best interests to administer employment tests rather than relying on the fact that an applicant has a high-school diploma. Whether someone has a diploma seems to be little evidence that they can read, write, think, do math, or do little more than be passed along by lazy administrators and teachers.