As part of the final in my freshmen computer class, they have to create a powerpoint presenting their "dream school." The quote above was the motto that one of my favorite students came up with for her school. A couple of points to make on this: she meant "impossible" because she spelled it correctly on a different slide. I pondered what she might've meant by 'Never underestimate the impossible' for a few minutes. If something is impossible, then I would say that we definitely haven't underestimated it.
Thinking of the 'impassible' as those students who refuse to put forth any effort and have zero regard for their grade, I actually like 'never underestimate the impassible' better! And it's true, they never cease to amaze me by their apathy.
Thankfully the 'impassibles' are still in the minority.