During the school day, if you go to the Great Hall and start dueling, you will have the same team everyday. However if you turn off your Gamecube, restart the game, then go to the Great Hall, you will face a different team from a different house.
The hidden Collector's Cards in Harry Potter and the Prisoner of Azkaban are good for more than poker. Each time you complete a set of five, you'll earn extra house points, stamina or other goodies. Note that you'll receive the fifth card in each set automatically when you find the other four.
This might be useful for people who don't know how to get rid of Dementors. Once a dementor catches you, use your control pad tog et rid of them. Note that if you get rid of them quickly you won't loose as much life as if it toke you long to escape from them.
If you want to upgrade your spells, switch to Hermione and visit Fred and George. Try to buy the page of a book for defense against the dark arts (not Folio Bruti) and Fread and George will propose you a challenge. You will have to search 9 statues of famous wizards and witches through the whole castle. Each time you find three statues, you can get one of the three book pages for free. Those pages will upgrade your spells. Note that only Hermione can list the statues once you find them. Good luck!
If doors are locked during the day don't worry I know a sercet... Go at night and all of the doors will be unlocked.
Use dungbombs to distract trolls and guards. You can also use flependo.