Immortalised by the Boomtown Rats’ chart-topping single I Don’t Like Mondays , the day in 1979 when 16-year-old Brenda Spencer killed two adults, shot and injured eight school children and blasted a police officer, remains a watershed event. Known as the first school shooter, Spencer’s crime in San Diego was unfathomable. But school shootings are now so frequent in the US that crime historian and broadcaster N Leigh Hunt says: “ School shootings are more common than weekends. CNN analysis says