GROTON (CBS) – A student ensemble is being forced to change its tune after someone complained that a Cathedral Concert planned for Easter Sunday Mass in Italy violated the separation of church and state.

The Groton-Dunstable Regional High School Chamber Chorus has performed all over the world, including at the London Olympics and even in medieval castles. The award winning group is used to being in high demand.

What they are “not” used to is having to cancel out on a gig.

“It’s a real shock to all of us, because I don’t think we’ve ever experienced anything like this,” one student said.

The chorus was chosen out of 800 other groups to perform at St. Anthony’s Basilica in Italy for Easter Sunday Mass. The trip to Italy and also Croatia was being privately funded, but the chorus had to bow out. The reason: A group threatened to sue them.

A former school committee member, Leslie Lathrop, contacted Americans United For Separation of Church and State.

That group wrote a letter to the school superintendent saying in part “Taking public school students to worship services as part of a public school activity is a flagrant violation of the Establishment Clause of the First Amendment of the Constitution.”

The school district decided not to fight it in court.

“The fear was probably that maybe we’re not in violation, but if it’s gonna be a long extended court case, the possibility of an expensive legal battle,” said Mike Woodlock, the school’s principal.

Members of the chorus WBZ spoke to are upset because they believe it has nothing to do with religion.

They pointed out they had performed at churches before and no one complained.

Leslie Lothrop knows people are angry at her. She denied suggestions she filed the complaint because she’s Jewish. “For a public school to do something that appears to endorse a particular religion, which this does, it’s just not allowed. It’s prohibited,” she said. “I’m just concerned the chorus adheres to what the statutes say.”

The group will still travel to Italy and Croatia in April for several performances.