Sergeant's earlier statement had essentially clarified that this was not a terrorist attack. Typically, terrorist attacks aim to claim major casualties to make a statement or someone announces responsibility, but this school shooting clearly didn't fit either category. Chris and Jimmy could have just informed their superiors and left; the NYPD was more than capable of handling it. Even if they had left and more casualties occurred, it would not be their concern—at worst, they'd simply be criticized for incompetence or inaction.
But since they were already there, Jimmy didn't want to just stand by and watch. Who knew how long the NYPD and ESU would take? Regardless of the shooter's motives, staging a shooting at a school was exceedingly vile. Delaying any further could result in more unfortunate students.
Having reached the third floor, Jimmy signaled to the officer guarding the staircase, then crouched and began to stealthily approach the classroom.