TL;DR, tell the police, not the news.
It was at some other school, so I’d probably just ignore it. However, if you have personal, or at least verifiable, evidence of a gun being present, the police are the ones that need to be called.
On that note, if you just heard this from someone else, it’s better left alone. A few weeks ago at my daughter’s school, there was a fight and someone mentioned that they were going to come back the next day with a gun. Facebook absolutely exploded on a handful of pages related to the school and city. For hours people were going nuts because they weren’t getting any updates from the school or the police. By 10pm that night, the school released a statement saying that there were multiple faculty/adults present breaking up the fight and there was no mention of a gun. Furthermore, the person who was said to be bringing a gun A)never said it and B)wasn’t part of the fight.
The police and school nicely, but sternly, told everyone they need to stfu about rumors that they have no knowledge of.
30% or so of the school didn’t show up the next day…because of a rumor that someone made up.
As I told my daughter both during and after this, this trainwreck is exactly why if you think someone has a gun or might bring one, you tell the first teacher, officer or parent you see. Don’t put it on facebook, that just makes it worse. I also explained to her that had she told me about this when she got home from school instead of 6 hours later when it was blowing up on social media, we would have gone to the police and/or called the school right away. It wouldn’t have even crossed my mind to call the news.