another probability paradox

One Friday a teacher told his class: students, next week there will be a pop quiz ("surprise" quiz on one of the 5 days Monday - Friday). To his horror, a few of the students were brilliant logicians - they consulted and told him that the quiz is canceled. Here's their reasoning:

As the quiz must be a surprise, it can't be on Friday, since if we got to Thursday evening and there was no quiz, it's not a surprise anymore. But then, can it be on Thursday ? It can't ! Say we got to Wednesday evening and there was no quiz yet. We saw why a pop quiz can't be on Friday. So it MUST be on Thursday. Therefore, it's not a surprise - therefore, there can be no pop quizes on Thursday.

It's easy to see how using the same reasoning we can prove that there can be NO pop quiz next week. Q.E.D.

What is wrong with the reasoning of these smart-asses, I ask ?