@PotomacBobGenerally, how the response looks like varies greatly based on WHERE your story takes place, and WHAT the call they're responding to told them. This goes all the way from simply rushing into the building, without a care for noise, right up to cutting the power to the building before entering it in full armor with cover on all sides.
Regarding the metal detector: AFAIK the most used model is the PD 6500i. It's "portable", so schools can move them around for events and such. It's also capable of being linked into a wireless network, so there's not necessarily someone standing next to it at all times. Especially after a call was made, chances are, the security is already at the scene. The machine can be remote controlled through the WiFi, so, if necessary, they can turn it off with a smarphone before walking through.
There are two ways to get past it:
1.) Press the "Off"-switch
That's a little tricky, because it's on the opposite side of the machine when you're about to walk into it.
2.) Pull/Cut the power plug
That's also a little tricky, because it's located on the top of the machine. It can be operated with a battery, but that would cost extra, so most schools don't.