Come to think of it, I've had personal experience over this whole "teach the controversy" thing. In my country we simply don't touch biological origins at high school level at all (a fact I've complained in my local newspapers about ;D), but in Form 3 (15 years old) we had to do a science project, and a Christian friend and I decided that we should try doing one on evolution vs. creation. So we went exploring all over the Internet and collected a whole bunch of creationist and evolutionist evidence.
Looking back, it was quite an unsuitable idea for two 15-year-olds, and it was an important factor that led me to support creationism for the next 3 or 4 years. We got a pretty crazy mixed bag of creationist PRATTs. Some of them were blindingly obvious, like that "evolution contradicts thermodynamics" one (and my friend had a fun time emailing AiG with an article showing that entropy can spontaneously increase on a nanoscopic level), but many were difficult to spot like claims that "the horse fossil series is false" or my personal favorite, the bombardier beetle is irreducibly complex.
You may argue that I should have been critical, but seriously: critical scientific thinking is not a very prevalent characteristic among teenagers, as far as I know. The typical 15-year-old interested in science is looking for the esoteric and the magical; to him, astronomy is often far more Star Trek or BSG than Hubble Telescope, and physics is always about quantum nonsense instead of the bread-and-butter of kinematics, cars and billiard balls endlessly colliding in absurdly friction-free environments, and the boring details of why buildings stay standing and what cement has to do with it. The schoolchild showing any interest in science is often into it looking for something to believe, not something to critique.
All this plays into the grand image of creationism as a shadowy movement of brilliant scientists, discovering new truths (spoken about in an ancient religious document, at that! How much more fantastical can you get?), and whose goal of spreading global enlightenment is only hampered by the persecution of an even more shadowy Evolutionist Conspiracy in the grand and foreboding ivory towers of universities and scientific labs worldwide. The combination of disrespect for authority, identification with rebellion, attraction towards the esoteric, and sheer ignorance can be deadly.
Creationism presented in the classroom may well turn into a Gish gallop impossible to deal with by normal pedagogical methods. After roughly two years of debating creationism on this forum I still have to learn new things and read more papers on many little details which turn creationist arguments on their heads. If it is taking me this long and this much work to refute creationism, and with me on the edge of obtaining a scientific education ... how much more difficult will it be with a roomful of teenagers more interested in science fiction than science?