An atheist organisation has been accused of bullying U.S. public schools by warning them that their students' trip to evangelist Ken Ham's Answers in Genesis Creation Museum in Petersburg, Kentucky would be unconstitutional.

The Freedom From Religion Foundation (FFRF) emailed letters to the Brookville High School in Dayton, Ohio; Jackson Independent School District in Jackson, Kentucky; and the Big Beaver Falls School District in Beaver Falls, Pennsylvania to point out that the trips would violate the U.S. Constitution.

"Public schools may not advance or promote religion. Bringing students on a field trip to a religious venue is a blatant promotion of religion," said FFRF Legal Fellow Madeline Ziegler in her letter to Nicholas Subashi, legal counsel for the Brookville Local Schools.

FFRF said the Creation Museum is a Christian homage to creationism with an explicit mission "to point today's culture back to the authority of the Gospel and proclaim the gospel message."

It said at the museum, there is a diorama of a human and a dinosaur together, implying that they existed simultaneously.

The FFRF said such trips to sectarian institutions also exclude the non-Christian and the nonreligious and the voluntary participation in the trip is not a valid defence in court.

"It is unconstitutional for a public school to take students on a field trip to a religious venue such as the Creation Museum, a Christian museum which promotes the religious doctrine of creationism and lists its mission as 'to point today's culture back to the authority of Scripture and proclaim the gospel message,'" the letters read, according to the Christian News Network.

FFRF said if the district is interested in planning an educational trip, it should be to secular museums.

It told Big Beaver Falls School District to "immediately cancel the planned May 20 field trip and refrain from taking young students on inappropriate, unconstitutional religious trips in the future."

However, Ham said it's not illegal for public schools to visit the museum.

"If public schools were bringing students here and their teachers were saying, 'THIS interpretation is the only truth that you should personally accept,' then that would be a violation of the Establishment Clause of the Constitution," he said.

However, he said, if the students visit the museum in an objective fashion and teachers show them the museum's interpretation of the origin of man, the trip is legitimate.

"Public school officials should neither personally endorse nor diminish the museum's view, but rather present our beliefs objectively," Ham said.

Ham said groups like the FFRF attempt to bully public schools because they don't want anyone to know about another interpretation of evolution.