Science is taught in schools because it it a powerful tool in exploring the world. Again, let me stress that science is not a set of conclusions, but a process. Science does, however, recognize that some ideas are better than others at explaining the available data. For example, the germ theory of disease best explains how some illness is transfered between different people. In fact, there is so much data to support this theory that it is accepted as the scientific consensus. These are the ideas we teach in a science class, ideas well supported by evidence. By and large, I don't think anyone objects to teaching science in schools, except when it contradicts their own ideology.
Just to be clear, I am not going to address the creationist arguments like flood geology and irreducible complexity here. I think that has been done exceptionably well all over the Internet, including at Talkorigins.org (which is a site I highly recommend). I just want to explain why their ideas are not science.
First of all, falsifiability is a basic requirement of a scientific idea. I have written about this before, but to recap: for an idea to be scientifically legitimate, there has to be a way to prove it false. Religious ideas, like creationism, are designed so that no matter what is observed, belief can be maintained. This alone is enough reason why creationism, in any of its various forms, can not be considered a scientific idea.
Second, creationism is not a single belief. Creationists (although intelligent design proponents will sometimes leave this out) often argue that creationism should be taught side-by-side with evolution; giving both sides equal time. This assumes that there is one form of creationism, when in reality there are as many different forms of creationism as there are religions. This was comicality pointed out in a letter to the Kansas School Board of Education. This letter says that alternative views of creation should be taught, including the belief that the universe was created by a Flying Spaghetti Monster. This is what started the Church of the Flying Spaghetti Monster, if you were wondering. If you want to teach creationism, you have to decide which one.
The reason I write this today is because of a recent article in the Telegraph. This article suggests that a more accurate translation of the Judeo-Christian bible reads "In the beginning, God separated the Heaven and the Earth," not "In the beginning God created the Heaven and the Earth." This brings up a huge point. One of the major differences between science and religion is that science always adjusts to new evidence. I would however, be shocked if die-hard creationists even give this new evidence a passing glance. Their beliefs are more important than the evidence that may contradict them, and this is justified through faith.
My goal is not to impose my beliefs on anyone else. Science is a strict and rigorous process. For any idea to pass as scientific, you have to build a scientifically valid hypothesis, and then come up with the evidence. Creationists have tried to skip this step. They are trying to use pressure on school boards and lawsuits to push their way into the science class. Science class is not for teaching ideologies. It is for exactly what the name implies: teaching science.