evolution IS religion. The question should be, why is ONE religion allowed (Darwinian evolution), but any other religious view NOT allowed in school.
That phrasing makes the real issue more obvious.
for more info, read what Dr. Gary Parker said (he authored five college biology textbooks while he was a believer in the religion of Darwinism, which he admits was a religion for him at the time). .
then he started to see that Darwinism is very unscientific and has NO basis in real science (it has no supporting evidence).
He said that while he was a biology teacher, he pushed his religion (Darwinism) in school all the time [and when he was a professor], and that there is no law against pushing religion in school, you just can't push the Christian religion in school, but you CAN push religion in school.
go to this link for an article in which Gary Parker shows how Darwinian evolution doesn't fit with science and therefore can't be true:
http://www.answersingenesis.org/home/area/cfol/ch2-selection.asp
Also, go here for more articles in the same vein by other top scientists:
http://www.answersingenesis.org
He has a very interesting testimony of the change in his life, when he realized Darwinism couldn't possibly be true and was just a false religion, instead of being real science.