Hermione Granger
Reasoning: I can't imagine a better character to fit this card than Hermione. This is the card of education, rules, following a discipline, conforming to a belief or group system, and also taking part in a cause or establishment. Hermione, of course, is one of the greatest students Hogwarts has ever seen. She tirelessly seeks to increase her knowledge, and share it with others. Hermione also shows a reluctance to break the rules, or defy the orthodox 'system', but will do so (and even enthusiastically so) when it is to further a cause she believes is right. Hermione has a highly-developed sense of social justice, devoting great energy to S.P.E.W., and is a natural leader and organizer of Dumbledore's Army. And of course, she is a prefect, and I expect she will be made Head Girl eventually, as long as the war doesn't get in the way.
On the negative side, this can mean blind and stifling conformity.
In a traditional tarot deck, this is the Hierophant.
Notes
I tried to give Hermione just a hint of a religious pose and 'poise'... this card often depicts a clergyman in a traditional deck, so I was hoping her uniform and the way she's holding the book and badge might lend a similar effect (but it probably doesn't to anyone but me. Oh well). The background was interesting... I'm not really sure what kind of bookcase that's supposed to be behind her, but it's nifty and looks like something Hogwarts might have. :B
The colors did NOT scan as accurately as I would like, and what's even more puzzling and irritating, it made the black of her robe look much less smooth than the original appears to the naked eye. Wierd.