Two shootings a month have taken place on average in US schools since the massacre of 26 children and teachers at Sandy Hook Elementary School in December 2012, research shows.

The killing of 20 six- and seven-year-old children and their teachers in Newtown, Connecticut, made headlines across the world, but despite the tragedy and demands for action, gun violence is still a very real threat in schools in the US.

The total of 28 school shootings has been revealed in an analysis by two gun control lobby groups: Moms Demand Action for Gun Sense in America and Mayors Against Illegal Guns. The number of shootings that affected educational institutions rises to 44 when universities and colleges are included. The incidents have resulted in 28 deaths - including seven suicides - and 37 non-fatal shootings. In just under half of cases, at least one person was killed.

According to Shannon Watts, founder of Moms Demand Action, parents can no longer be certain that their children will be safe at school. “Parents in communities across the country live in fear,” she said. “An estimated 90 per cent of school districts have tightened security since Newtown, installing metal detectors, surveillance cameras and bulletproof glass.

“Schools now routinely have lockdown drills reminiscent of Cold War air- raid drills.”

Among the organisations that have been campaigning for tighter security in schools is the charity Safe and Sound, which was started by the parents and wider community directly affected by the Sandy Hook shootings.

Michele Gay, who lost her daughter in the tragedy, told TES that schools needed to use a “multi-layered approach” to counteract the possibility of an intruder entering a school.

“With security top of mind for so many communities today, we advocate for multiple layers of security,” Ms Gay said. “The national average response time in an emergency in the US is between five and six minutes. No one measure or practice can ensure safety. But a layered approach, involving many simple interventions, can save lives and enhance safety in those precious minutes before help arrives.”

Safety measures could include quick-locking classroom doors, protective film over windows to prevent access, ensuring that schools have just a single entry point and training all staff to deal with an emergency.

A spate of incidents took place in the US at the start of the year, with 13 school shootings in the first six weeks of 2014 alone. In a period of just eight days, four shootings took place on school campuses.

The number of shootings has led parents and politicians alike to call for tighter gun controls, with Connecticut senator Chris Murphy asking the country’s lawmakers to take action to prevent further gun violence.

“No parent should have to fear for the life of their child when they drop them off at school,” he said in a statement after the publication of the figures. “We can do more to prevent these tragedies, and it’s time for Congress to wake up and do the right thing.”

Despite demands for legal reform, however, little progress is being made, and this has forced schools to take action themselves by boosting security. Last year, TES reported that states across the US were considering or had introduced laws that required schools to carry out regular “lockdown drills”, similar to fire drills. Some had even held so- called “active shooter drills” - Penn High School in Indiana carried out a mock shooting complete with fake blood and a body count.

Marianne Alvarez, a specialist in training school staff to deal with intruders, said that police forces needed to help schools to be better prepared for emergencies, in the same way that fire departments did. She acknowledged that the problem of school shootings was “growing” in the US, adding that there was “no end in sight” to the rising level of violence.