It's pretty simple. Each year the media focuses more and more on these stories. Doesn't mean there is a bigger percentage of issues, just a better percentage of media coverage vs. number of issues.
What you will not see, because it doesn't get headlines or raise ratings, is a story about the majority of public schools that don't have issues and do include Christmas every year.
IMO it's no different than the other headlines and issues. You can't take the number of stories as op ed pieces and translate those into sings of a major epidemic. In fact, on many occasions the same people going off about this issue could be the ones ignoring other media hot buttons knowing it's all about the ratings.