High schools in the district of Huntsville, Alabama, have been monitoring their students' activities on social media for the past 18 months after receiving a tip from the National Security Agency (NSA) about violent threats being made on Facebook.

Huntsville schools superintendent Casey Wardynski told Alabama local newspaper conglomerate Al.com that Huntsville City Schools started scanning Facebook and other social media websites for signs of gang activity as part of a monitoring programme called Students Against Fear (SAFe) after receiving the tip.

The NSA denied these claims, while members of the Huntsville school board said they were not made aware of the programme.

NSA denies call to school district

"The National Security Agency has no record that it passed any information to the Huntsville school district and the description of what supposedly occurred is inconsistent with NSA's practices," said Vanee Vines, public affairs specialist with the NSA, in a statement.

"Moreover, NSA does not make recommendations regarding school safety programmes."

Documents obtained by AL.com dated 19 March 2014 show four different students posing in photos on Facebook with handguns. Although none of the photos appear to have been taken on school grounds, the documents state three of the students were expelled and the fourth referred for counselling.

Al Lankford, a long-serving school security officer, told the newspaper he took the phone call from the NSA and the information from the call led to security officers visiting a high school and searching a male student's car.

"We found a very good size knife and the student was expelled," confirmed Wardynski, a former US Army colonel appointed as superintendent in Huntsville in 2011.

Wardynski said the NSA contacted the Huntsville school district due to a "foreign connection", as the student who had been making violent threats on Facebook had been found to be chatting online with a group that included an individual in Yemen.

Principals advised to photograph graffiti

The presentation materials state the SAFe programme has been successful in identifying students who pose a safety threat in all high schools in the Huntsville district, and the documents also advise principals to photograph school graffiti before removing it, as "gang graffiti has potential intelligence value to HCS Security and law enforcement".

A new privacy bill, AB 1442, has been passed into legislation in the state of California and is only awaiting a signature by the governor before it is enforced.

The bill will ensure parents are informed when their children's social media activities are being monitored and ensure any data collected by school districts cannot be used against an individual years after they cease to be a minor.

In the meantime, however, school districts in other states are free to use controversial services such as Geo Listening, a startup that monitors social media posts to better "meet the social and emotional needs" of middle and high-school students, including to spot cyber-bullying, suicides and drugs.