Officials at the National September 11 Memorial Museum in Manhattan said on Monday that one of their security guards should not have stopped a North Carolina middle school choir from singing the national anthem on the plaza last week.

“The guard did not respond appropriately,” Kaylee Skaar, a museum spokeswoman, said. “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 memorial on Wednesday and had just started singing “The Star-Spangled Banner” when a guard told them to stop.

Martha Brown, a teacher from the school, said on Monday that a different security guard had given permission for her students to sing. But the second guard said, “You just can’t do this; you’ve got to stop now,” Ms. Brown said. “So we very reverently and quietly stopped what we were doing and complied with his request and quietly exited the park.”