According to choir director Martha Brown, another guard had already given the students permission to sing, but a second guard told them to stop.
They were told a permit is required to perform at the memorial.
Brown told IE: “I was a little shocked and the patriot in me (was) a little bit angry. Once he explained, I understood.”
In a statement, memorial spokeswoman Kaylee Skaar said, "The guard did not respond appropriately. We are working with our security staff to ensure that this does not happen again with future student performances.”
Brown told IE was "so proud of these students” because they responded to the guard's demand with respect and dignity.