Paradox of the surprise test

A teacher announces that he will apply a surprise test over the next week.

But students know that the test cannot be applied on Friday, the last day of the week, because then it would not be a surprise.

Thus, Thursday becomes the last day the test can be applied. But if the teacher waits until Thursday to apply the test, it would come as no surprise.

Similarly, each day of the week can be eliminated, and the surprise test can never be applied.