Two students sued the Boulder Valley School District on Monday after they were denied permission to start a Bible club at Monarch High School.
Ashley Thiele, a senior, and Amy Duvall, a freshman, proposed a club at the Louisville school that would "teach students biblical principles that will help them in life."
The proposal was turned down in November by Superintendent George Garcia.
In a letter to the girls' attorney, Garcia cited a policy limiting clubs to subjects related to the curriculum.
The lawsuit filed in federal court in Denver claims the school district permits numerous other clubs that are not related to the curriculum, including the Gay/Straight Alliance and the Multicultural Club.
Allowing those organizations, but not the Bible group, is discriminatory, the lawsuit charges.
The lawsuit says the Bible is relevant to such areas of the curriculum as history and literature.
A school district spokeswoman declined to comment, saying officials have not seen the lawsuit.
But in his November letter, Garcia said the other clubs cited by the students are, in fact, related to the curriculum.
For example, the Gay/Straight Alliance is "a product of our health education," Garcia said. The Multicultural Club is part of a class on diversity.
Garcia said the curriculum includes references to the Bible as part of classes in history, literature, art and music.
But a club dedicated to teaching religious values would "intrude into (the) area of personal and spiritual importance reserved for the family."
Margot Duvall, the mother of Amy Duvall, said the families have been told by lawyers not to comment on the lawsuit.
The students are represented by the Virginia-based American Center for Law and Justice.
"The law is very clear on this issue," said Stuart J. Roth, senior counsel for the group.
"If a school district permits other student organizations on campus, it cannot legally deny students the opportunity to meet because their message is religious."
The lawsuit names as defendants the Boulder Valley School District, the Board of Education, its president and members, the superintendent of schools and the principal of the school.
The lawsuit contends the defendants violated the First and 14th amendments to the U.S. Constitution and the Equal Access Act.
The lawsuit requests the court to declare the actions of the school district unconstitutional and asks that the court grant an injunction prohibiting the school district from continuing to engage in the alleged discriminatory action.
The Associated Press contributed to this report.