well, the robes i understand. they're in an extracurriculer activity and the robes get in the way. if harry was smart, though, he'd realize that they should practice with the robes for that very reason. most likely, the robes are an ugly visual for close scenes with a lot of people and the costume designer and/or director didn't want them.

but pants? even if they were a hand-me-down, schools with a uniform usually frown on the break of the uniform. even if it is part of the boy's uniform, one girl can't wear pants or they all would want to.

(btw, i think it's cool. i feel sort of chauvinist, but it's just... odd.)