The scene is an empty classroom. Gun casings litter the floor. On the blackboard, someone has written a quote by George Santayana, a Spanish-born American philosopher and author of the late nineteenth and early twentieth centuries: "Those who cannot remember the past are condemned to repeat it."
The quote means that studying history is necessary to avoid repeating past mistakes (the First and Second World Wars, for example). The cartoonist plays on the phrase 'history lesson': history lesson at school, and the lessons to be learnt from the study of history. The message is that America seems to have learnt nothing from earlier school shootings (Columbine and Virginia Tech being the worst tragedies in recent years).