The two reasons I don't think ID should be taught in class are practical (though I agree with everyone's above posts)
First - how do you grade students, when the correct answer to every question is "I dunno, God did it." Everyone is always going to get an A+ on every test by pleading stupidity.
Second - How do you determine which version of ID/Creationism you use. In other words, do you say "There was an intelligent force" and stop there, or do you say, "God did it", or "Thor did it" or "ET did it"... Is the Jewish God the God of creation, or is it any one of the thousands of God's that predate him/her/it?