A New Hampshire school district has been asked to change its policy after the superintendent ordered the removal of the word "Christmas" in a privately produced flier to inform the community about a "Christmas tree" lighting event.

Superintendent Robert Malay of the New Hampshire School Administrative Unit (SAU) 29 told John Fletcher, commander of the local American Legion, to change the word "Christmas" in "Christmas tree" to "holiday" in the flyer, which was distributed to advertise Marlborough town's annual Christmas tree lighting.

Fletcher has been distributing the identical flyers except for the date of the event at Marlborough School for years.

The SAU 29 allowed the distribution of private flyers but said the flyers "must be secular."

In its letter sent to the school district Wednesday, the Alliance Defending Freedom (ADF), a conservative Christian non-profit organisation, said the district's decision was unconstitutional.

"Simply put, using the word 'Christmas' in a flyer advertising a 'Christmas' tree lighting and the appearance of Santa Claus [are] only sensible and plainly not unconstitutional," wrote ADF Senior Counsel Jeremy Tedesco.

He explained that "calling Christmas trees what they are is not unconstitutional. But it is unconstitutional to censor private speech simply because it is religious in nature."

In protest, Fletcher distributed the fliers with white-out covering the word "Christmas" instead of using "holiday" as a substitute word.

In his letter to the school board, Malay insisted that the title "Annual Christmas Tree Lighting" must be changed to "Annual Holiday Tree Lighting."

He said this decision was prompted by concerns about the Establishment Clause.

However, the ADF said "the Establishment Clause requires no such thing. Courts have long recognised that 'the vigilant protection of constitutional freedoms is nowhere more vital than in the community of American schools.'"

In its letter, the ADF told the district that its "actions impermissibly target religious speech for censorship and constitute illegal viewpoint discrimination."

"In fact, SAU 29's policy explicitly singles out religion in a forbidden manner, stating that flyers 'must be secular.' This fosters 'a pervasive bias or hostility to religion [that] undermine[s] the very neutrality the Establishment Clause requires," it said.