Once schools start getting out, that's my cue to scoot on out. ANY time schools are out (even some) crowds start picking up. Last year a lot of schools started releasing on the 16th, and the crowds went right up along with it. They build over that time, getting particularly bad the couple days before Christmas, then peaking between Chistmas and New Years.
So yes, the week before Christmas is less busy than the week after Christmas, but that very last week is indeed pretty busy.