Immediate entry and press the fight.
What we're seeing time and time again is that active shooters are typically offing themselves when presented LE confrontation. Either that or barricade themselves into a room (then off themselves). But typically when cornered they decide to take the easy way out. (At least that's what we've been seeing.)
Worse case scenario you lose an officer and the threat stays active. Second worse case scenario is he barricades himself in a room with a lot of hostages. The latter is still probably better then him continuing the spree because at that point you have containment + possibility of negotiations + hostage rescue tactics at work.
Pre Columbine it was wait for three but you try that crap now and your done in LE, at least, around here. On average someone dies what, every 3 seconds? Every gunshot you hear is a body. Congrats you stood outside with guns, ammo, and body armor while a bunch of innocents got killed. Its hard to fault the guys at Columbine because thats what they were trained to do. But lets be real, if any of those officers had kids in there I wonder if they'd have stood outside and waited for backup VS making entry.