Ms. Leena, an art teacher for an 4th-grade elementary class, begins her lesson instructing her class to pair up and to begin to paint one another's portraits. This assignment creates a conflict between student's Anthony and Ricardo, as the former refuses to associate with his partner. The conflict escalates and Anthony is led outside of the classroom to have a discussion. While Ms. Leena is speaking with Anthony, a teenager is seen running towards a nearby classroom and spray-painting an 'X' on the classroom door, after which he hastily leaves. At this peculiar event, Ms. Leena ends her talk with Anthony, telling him to rejoin the class, and proceeds to call a co-worker to report the suspicious behavior. After her phone call ends, she grows even more concerned as she realizes her classroom door is also marked similarly, with a spray-painted 'X'.Ms. Leena rejoins the class and begins to clean the paint that was painted on Ricardo by fellow classmates. However, she is interrupted by the sound of blaring alarms. Locking the door, turning off the lights, and closing the curtains, the class goes into an urgent lock-down situation. While doing the best she can to keep her students quiet and pacified, Ms. Leena witnesses outside of her window two young teenagers putting on armored vests and distributing guns. The two gunman proceed towards doors with spray-painted 'X's, one being the most recently marked. Gun shots and screams are soon heard as Ms. Leena begins to barricade her own door. Realizing the safety of herself and her class is at increasing risk, she has her students move to a nearby classroom that a gunman has just left, though not before realizing one of her own students is missing.