Law of Supply
Here is a wonder article from the BBC which helps teaching the Law of Supply to your AS Economists or indeed your IB sets. There is also a video clip here which shows how wet weather can also affect crops of potatoes.
You could tweak this Q&A article for a homework and use graph to help demonstrate why the volitity of the price of wheat effects us all. A great table at the bottom of this article shows the concept of over or under production and it's consequences.