Post-modernism actually defies easy definition even among its adherents, which makes it nearly impossible to discern what the average Florida legislator thought she was doing when she voted for this law (my definition is that post-modernism represents a critique of meta-narratives). But the supreme irony is that Florida's account of what American history "is", is in fact quite relativist in its own right:
The history of the United States shall be taught as genuine history and shall not follow the revisionist or postmodernist viewpoints of relative truth. American history shall be viewed as factual, not as constructed, shall be viewed as knowable, teachable, and testable, and shall be defined as the creation of a new nation based largely on the universal principles stated in the Declaration of Independence.
I think it is rather fanciful to say that the new nation was based even "largely" on the principles of the declaration of independence. The truth-value of that statement is relatively high from the perspective of landed White male, and relatively low from the perspective of nearly anybody else. Ask any woman, Black person, Native American, or even landless White male what they think of that statement. I pretty confident that we've hit well over 50% of the population with those groups. Manifest Destiny? Trail of Tears? Slavery? Jim Crow? Japanese Internment? America was and remains a work in progress, and the universalist principles undergirding the declaration have not been achieved even today, let alone at the founding.
I can't get Richard Rorty out of my head: "Truth cannot be out there - cannot exist independently of the human mind - because sentences cannot so exist, or be out there" (Contingency, Irony, Solidarity at 5). The way we describe the world is a construction, so how can we pretend like we are uncritically reflecting a world that is out there? I remember learning about both the Battle at Wounded Knee, and the Massacre at Wounded Knee. Which is it? To say that these constructions are just academic playthings is just naive.
Of course, as Levinson notes, we could just bar David Hume and Thomas Kuhn and nearly every development in modern philosophy from Mill on out. That would significantly disadvantage the academic progress of Florida students. But I've gotten the distinct feeling that Republicans are actually hostile to academic achievement--since they think that higher education itself is biased against them. Well, maybe it is--colleges and universities have pretty well bought into the principles of equal humanity and dignity that the GOP (with their lovely FMA) continues to war against. But they really can't pretend to be pro-education at the same time as they try and sabotage it.