The Douglas County School District in Colorado is being sued by humanists over its participation in Samaritan's Purse's Operation Christmas Child initiative.
The scheme asks members of the public to donate a shoebox filled with gifts in the run-up to Christmas that are then distributed to children in impoverished countries. Donors are allowed to enclose a personal note to the child recipient of their shoebox.
The lawsuit has been brought against Douglas County School District by the American Humanist Association, which claims "multiple violations" of the Establishment Clause of the US Constitution.
The AHA said the district was guilty of the "repeated practice of promoting and supporting programmes that proselytise Christianity", including Operation Christmas Child.
The lawsuit also challenges the partnership between some schools in the district and two other Christian organisations, Adventures in Mission and the Fellowship of Christian Athletes.
Activities the AHA objects to include promoting a mission trip to Guatemala that was attended by two high school teachers and advertised using school resources, and the attendance of school staff at blessings and worship services at local churches prior to mission trips.
"This public school district seems to be on a mission to promote Christianity, using taxpayer dollars for evangelical and missionary purposes," said David Niose, legal director of the Appignani Humanist Legal Centre.
"As state entities, public schools must educate students in a neutral environment, not push one particular religious belief."
Roy Speckhardt, executive director of the American Humanist Association: "By pressuring public school children into participating in programmes that proselytise Christianity, the school district is marginalising religious minorities and students with no religion at all."
