It is interesting to note that the legality of a school board's action, in part, depends upon their intent. In this case, the school board was clearly motivated by a religious belief in creationism to include a warning against the Theory of Evolution and to promote Intelligent Design Creationism.
How would the case have faired if the board had listened to ID proponents as well as scientists and decided that it would be in the best interest of children to learn about both ideas? One of the Defense expert witnesses, David Fuller, who is no religious fundamentalist, thought that ID should not be excluded from the classroom because it might spark interest in children to persue science. He notes that many scientists have been inspired by religion and the belief in God. If belief in God has been shown to inpire an interest in science, could teaching ID have a secular purpose of motivating children to learn? This is an interesting argument, but one could just as easier argue that Protestantism should be taught in schools because it encourages a strong work ethic, or you could argue that a law forcing everyone to attend church is secular because reminding people about God's love encourage lawful behavior. That religion can inspire is not enough to justify the government proselytizing. I think there is an important distinction between a religious motive to explore science and religious beliefs attempting to be labeled science.
I think the easy answer to the question above is that it would never happen. A thoughtful person who honestly wanted the best education for children would not present ID in a classroom other than a discussion of logical fallacies, or in a discussion on the philosophy of science, or Epistemology in general.
While they might not have liked it as much, the Dover board could have used the National Academy of Sciences statement on science
Science is a particular way of knowing about the world. In science, explanations are limited to those based on observations and experiments that can be substantiated by other scientists. Explanations that cannot be based on empirical evidence are not a part of science.
They could go on to say that "explanations that are based on a faith, such as a belief in God, are not science because they are not based on observations and experiments that can be repeated by others. Science does not make a judgment about the supernatural, or knowledge based on belief in the supernatural."
This gets them half-way to where they want to go, and would not run into any legal problems.
The ID movement is a group of disjointed arguments most of which are distortions, misunderstandings, and lies about evolution; other arguments are simply poor analogies. The only bit I would consider serious a somewhat interesting (and a plausible basis for a legal challenge) would be the attack of evolution as an attack on the philosophy of science. As noted above, science does not, and can not, investigate claims of supernatural events. It begins with idea that (without any physical, measurable evidence) there is no God intervening in human affairs. If God came down and healed and created under controlled conditions, God would be a part of science, but as it is He remains mysterious. Thus, ID'er claim science assumes no God exists, just as much ID assumes God exists. Since both implicate religious beliefs, either both or neither should be taught in schools. However, this attack is against all science and not just evolutionary theory. To accept this attack is to doubt the amazing predictive power that science has been shown to demonstrate. One of the reasons the Dover board lost their case was that the statement about evolution being merely a theory, not a fact, focused on evolution alone, singling is out as a subject to doubt. I think this focus on evolution exposes the attack on the philosophy of science as just a convenient argument.