The shooter broke out the second floor window to use it as a overlooking sniper position against officers arriving outside.
Fortunately, the first officers to arrive had already formed the entry team and had entered the school building's first floor.
The first officer to arrive was immediately met outside by a school employee, who provided a master key allowing the entry team immediate and quiet access.
Also fortunately, the shooter neglected to watch for a flank while firing from the second floor overlook position on officers outside.
The officers outside did not engage, they drew fire by simply being in view and attracting attention, exactly what was needed to distract the shooter.