They Cheated

Once two star football players had failed a test, and could not play football in the championship game.
So, after much begging from the coach, the teacher finally let the two take the test again.
They took the test, and turned it in.
The coach and the two students watched carefully over the teacher grading the tests. She checked over the first test, then over the second test. Half way through the second test she stopped and put a great big 'F' on both tests.
The coach was furious and demanded an explanation. She said that they had cheated. 'Why?' the coach asked.
The teacher showed him number six. The coach looked at number six on the first test.
The answer read 'I don't know.' The coach said that it did not prove anything.
The teacher handed him the second test. The answer read 'I don't know either.'