Say 500 kids in the school have phones. 250 of them call 911. The vast majority of emergency call centers have zero chance to deal with that volume. So all the emergency responders are caught up on the lines sifting through children while the adults with the most useful information are stuck on hold.
Let's say the same number of kids call their parents, either before or after they try 911. 100 of those parents drive directly to the school in 100 different vehicles, coming from both directions on what is probably a two-lane road. Now the crime scene with police needing to get in, ambulances needing to get out, etc. is completely congested with terrible traffic that slows everyone down, and police officers who should be focused on dealing with the shooter and saving kids are instead focused on crowd control.
Meanwhile, within the school, teachers and other staff are trying to manage the situation. They need children to be paying absolute attention to them and following their instructions immediately. But instead, half their kids are on the phones obsessively tracking news and social media or sobbing to their parents.
A teacher in every classroom means you have a phone in every classroom, plus in the offices, plus on staff moving through the hallways. That's more than enough phones.