That's not really an easy solution if your call is just one of 20 or 30 coming in. What makes your call more valid than the rest? I am reminded of the 6 blind men describing the elephant--each can only describe the part of the elephant that he can perceive. An active shooter situation in a school is the elephant and with the prevalence of personal phones, there will most certainly be more than 6 blind men describing only the portion of the situation of which they are aware. The various descriptions will almost certainly be a contradictory and confusing mish-mash of facts and imagination. We saw this with the initial calls from Sandy Hook and it happens to varying degrees anytime there are multiple complainants on the same incident.
Telling the 9-1-1 operator "I AM A CIVILIAN AND I AM ARMED" will probably get you through the dispatch and what they say may get past the random noise and radio chatter. 99.999% likely no one else who calls it in is going to be armed. Telling the 9-1-1 operator where you are at, that you are armed, number of shots fired that you've heard, number of gunmen, where they are at.. it'll get relayed.
Also my advice wasn't "school" specific. Valid for any encounter at home or away. (Guns are prohibited in IL on school grounds for CCL holders, so at least in my state, it's a moot issue anyway).
Concealed carry holders aren't in the majority by any means, and those who actually carry daily are even more remote than # of permit holders. Being a CCW who suddenly finds him/herself in an active shoot scene is kind of like getting struck by lightning twice; the stars have aligned for you, for better or worse.
Still on the most important point, we agreed; what you do when police are THERE, and on-scene, is incredibly important. in NRA personal protection classes there's a LOT of dialog about "what to do when police arrive." (Also covers what to say on the phone with police dispatch .. for what it's worth)
Giving good information on the phone is also very important so police aren't going in totally blind; declaring that you are a civilian who is armed with a handgun is important information.
After all, the COPS coming in, need to declare who THEY are as well, because you are still in defensive mode until you are able to successfully egress the situation.
(Again, in PPITH where we teach if you barricade yourself, the rule of thumb is to make sure and verify that it is really cops trying to get in to your room, and not a bad guy who is SAYING he's a cop...)
Staying on 9-1-1 for the duration of the event is important, if you are in immediate danger. Even if you put the phone down leave the line open so what happens is recorded.