Continuity mistake: In the scene where they are having the finals, the things on the desk change a few times.
Other mistake: When London is reading her book to the little children, the small girl wearing pink with a ribbon in her hair is miming what London is saying, just before London actually says it.
Plot hole: The whole episode is based on false facts. Cody was getting an A in woodwork and Zack was failing in all his classes the previous year at school (according to their report cards in Ep "Smarter and Smarter"). So Cody knows a lot more about woodwork than Zack and should be able to make a clock, never mind a birdhouse, and Zack should not be able to make a clock.
Answer: The show never says.
Brad ★