New York - Officials at the September 11 memorial said on Monday that one of their security guards shouldn't have stopped a North Carolina middle school choir from singing the national anthem on the memorial plaza."The guard did not respond appropriately," said the spokesperson, Kaylee Skaar. "We are working with our security staff to ensure that this does not happen again with future student performances."About 50 students from Waynesville Middle School in western North Carolina were at the 9/11 memorial on Wednesday and had just started singing The Star-Spangled Banner when a guard told them to stop.Teacher Martha Brown said on Monday that a different security guard had given the OK for her students to sing. But the second guard said, "You just can't do this. You've got to stop now," Brown said. "So we very reverently and quietly stopped what we were doing and complied with his request and quietly exited the park."Video posted on Facebook by an adult on the school field trip sparked outrage and led to an invitation for the students to sing the anthem live on Fox News. Brown and Principal Trevor Putnam joined the students for their performance at the school on Monday.Brown said her students learned from the experience. "We turned it into a teaching moment and taught them that even if you don't agree with it, or understand it, you must respect authority," she said.Putnam echoed the sentiment."The lesson learned here is always to respect authority," the principal said in a telephone interview. "And I'm so proud of our kids for conducting themselves the way they did."Crystal Mulvey, a parent who accompanied her daughter on the school trip, said she was shocked that the guard interrupted the national anthem because "it's kind of a sacred song to us". But she added: "On the flip side, I completely understand following rules."Striking a balance between remembering those killed by terrorists and rebuilding a bustling commercial district has been a challenge ever since the twin towers were toppled in 2001. Tourists paying their respect to the dead now share the memorial plaza with neighbourhood residents and office workers from the new World Trade Centre towers lunching under the trees.September 11 memorial officials say groups wishing to perform on the crowded plaza are supposed to pay $35 to apply for a permit and obey a list of rules, such as no amplified sound and no interfering with the flow of traffic. Brown said she was not aware of the permit requirement.Visitors to the memorial had mixed feelings about the rules."To me, the children's singing was in remembrance of people who had a soul. It was reverence for their lives so they're not forgotten," said Gwendolyn Tucker, herself a security guard from Los Angeles. But, she added, "the guards have to keep it orderly".