A 17-year-old boy with a loaded handgun tucked inside his waistband was busted at his Brooklyn high school Monday morning, authorities said.  The teen was packing the loaded 9 mm handgun inside East Brooklyn High School on Kings Highway near East 96th Street in Flatbush just before 10 a.m., cops and sources said.  Another student spotted the firearm and quickly alerted teachers, who called 911, according to the sources.  The gun was recovered and the boy was taken into custody at the scene,