We went May 2012 from the 5th - 12th and I'll echo the pp's advice, avoid Friday at all costs and Saturday is also crowded. Sunday is probably you're least crowded day especially if you start early. Monday-Thursdays were not bad either just try to arrive early especially for the Harry Potter area.
As far as the Friday field trip crowds are concerned, we didn't find the kiddo's had bad behavior or anything there were just TONS of them and of course they're excited, loud and acting like kids.