The School of IT at the University of Sydney is running a python programming challenge for high school students over 5 weeks in term 3, 2009 - starting on Monday 10 August.
The NCSS (National Computer Science School) Challenge is designed to cater for both beginners and advanced students. Each week, a set of Python teaching resources for either in-class or self-directed learning will be distributed to participants via email and online. A set of challenge questions testing this material will also be distributed. Each week's challenge set will range from relatively easy to extremely challenging allowing beginners to progress at their own rate whilst extending gifted students. The challenges will increase in complexity as more and more programming concepts are covered over the 5 weeks.
Teachers can enter too. I have participated on the past two years and wrote a review two years ago. Note that the competition has changed since then; it didn't have the beginners and advanced sections back then.