On the second day of school, go to the Quidditch practice. Complete the training with a "B" or better rank to get the Nimbus 2000 flying broomstick. You can now fly anywhere around the school during the day. The catch is you can only land on grass and not fly inside the school.
We have loads more cheats and hints for this game in the user submitted sections.
Just follow these links to see user submitted cheats and hints respectively:
User Submitted Cheats
User Submitted Hints