So why not teach ID in schools?
Because of this (from http://www.creationtheory.org/Essays/Phrases.xhtml#sdfootnote1sym):
Intelligent Design ("ID") Theory
Non-fundie definition: “Creationism in a clown suit!”
It’s just creation with "God" replaced by the more vague "intelligent designer."
Fundie definition: “The theory that because science does not yet understand everything in the universe, God an "intelligent designer" magically created all life.”
ID draws its strength from the idea that if we don't know exactly what a particular species' or feature's evolutionary precursors looked like, then there must not have been any, so it must have appeared spontaneously at the will of God.