You can read details of the incident here: http://www.wired.com/wiredscience/2010/04/evolution-controversy/ and http://www.acorn-online.com/joomla15/thewestonforum/news/local/56145-evolution-flap-ribbens-apologizes-weston-parents-have-questions.html.
The official excuse was that evolution is “a philosophically unsatisfactory explanation for the diversity of life,” and is “not age appropriate, is not part of our existing curriculum, is not part of the state frameworks at this point in a student’s education, nor a topic in which you [teacher Mark Tangarone] have particular expertise.”
The claim that it was not part of the state frameworks turned out to be false (http://is.gd/bDLT3). The principal and teacher have since resigned and the principal later apologized for his decision.
The anti-evolution extremists have gotten administrators so terrorized that they are afraid to teach evolution. The idea that young children will be traumatized by learning about evolution is propaganda and refusal to teach evolution on this basis will produce the exact result it allegedly tries to avoid. Children are naturally curious and curiosity about human origins is a natural and expected development. When this curiosity is not satisfied with facts that explain the prevailing consensus, they fill the void with whatever other explanation is offered. This is usually the pseudo science of intelligent design or blatantly religious creationism put forward by adults with religious motivations for their ideas and little knowledge of biology. Children grow up accepting these ideas, and when they are later presented with evidence for evolution, they are forced to reject the authorities they've trusted in order to assimilate the information. This leads confused children who complain to their parents, who then complain to the school boards, who respond by watering down text books and "teaching the controversy".
A stand must be taken and school boards need to teach the community, not just the students, about how science works and the preponderance of evidence for evolution. If parents still don't want to face these facts, they are free to home school their children or send them to private schools which cater to their prejudices. Public education needs to stick to the facts and let children know what scientists already know, that evolution is fact, not speculation.