A New Jersey public school has barred students from saying "God Bless America" during the daily morning assembly after the American Civil Liberties Union (ACLU) sent a letter telling school officials that it is unconstitutional.

Every morning for the last 14 years, students at Glenview Elementary School under the Haddon Heights School District would recite "God Bless America" after the Pledge of Allegiance to honour the first responders after the Sept. 11, 2001 terrorist attacks on the U.S.

"It never, to us, invoked any type of religious intentions. It was basically a patriotic gesture that the boys and girls were doing," Principal Sam Sassano told KYW Newsradio, according to CBS Philly.

But the ACLU-New Jersey sent a letter to the district on Dec. 30, 2015 to say that it is unconstitutional.

"I write to inform you that engaging in a school-sponsored practice of having elementary school students invoke God's blessing at the beginning of every school day, during an official school assembly, is unconstitutional. I am hopeful that this matter can be quickly and amicably resolved," Legal Director Ed Barocas wrote in the letter.

ACLU-NJ said reciting "God Bless America" violates the Establishment Clause of the U.S. Constitution, which prohibits government sponsorship of religion.

To avoid an expensive legal battle with ACLU, Sassano sent an email to parents to say that the school will look for other ways to honour the responders.

Barocas claimed that "our Constitution is clear: schools can't coerce or impose religion on children."

"It's the job of parents to decide how and whether to instil religion, not public schools. There is a special concern when it involves students at such an impressionable age, including kindergartners," he said.

The letter states that "the phrase 'God Bless America' calls upon God to bless our country in a more shorthand form than the phrase used in Engel v. Vitale, but the basic result here is the same: a daily state-sponsored practice of reciting—in a way that prompts student participation—a phrase that invokes God's blessing. The fact that the phrase has some patriotic overtones does not sufficiently negate its fundamentally religious nature."