The timeline on the DVD *confirms* canon.

[Sally]

No: HPFGUIDX 55856

I just thought I'd throw this in - my birthday is a week before 
Hermione's, and I was the second youngest in my graduating class. My 
best friend was a full 10 months older than I, and in the same grade. 
Neither of us failed or skipped years. I seem to recall that kids 
with birthdays that close to the cut off have to be tested before 
they're let in. But I'm American, so I'm not sure how much sway that 
holds.

Personally, I like to think she's younger than the boys - it makes 
her being the top student in their class that much more impressive :)