The first 2 weeks of December used to be very slow times for park attendance, because as you say schools are still in session. Those 2 weeks also used to be some of the cheapest for resort stays, both for
DVC points per night and for cash reservations. That combination of lower crowds and cheaper rooms made those weeks very, very popular, which translates to harder to book. Many people visit who aren’t tied to the school schedule!
Now that DVC has reallocated points so that those two weeks are more expensive points-wise, in a few years we’ll be able to determine whether demand has changed as well.