While many people are familiar with the Declaration, I would wager few have actually read it, outside the Preamble. The Declaration begins as a noble-sounding document full of many seminal phrases including "When in course of human events...", "All Men Are Created Equal", "Life, Liberty, and the Pursuit of Happiness" , and the rest. And yet, beyond the first paragraph, the document becomes a bit more political. In fact, the bulk of the document is a list of complaints against the king and, while the complaints may well be valid, it's easy to see when reading the declaration with fresh eyes, that it's less a noble statement of truth, but also filled with propaganda. It turns out that the Founding Fathers have been politicians this whole time.
While the students were a little surprised and perhaps even disappointed, I think the larger point is that propaganda is not inherently positive or negative...but rather it depends on the intention. In the wrong hands, its absolutely evil, but if used for a greater good, it can eventually be enshrined in the National Archives forever.