Everyone basically took it for granted that it must be true that Sidwell Friends School, the school in question, has armed guards. After all, the President's daughters go there, as do the kids of a lot of other high profile people in DC. So it does seem sort of plausible. And why would they say it if it weren't true? Famous last words.
But it turns out the whole thing was a lie. Glenn Kessler, the Washington Post's factchecker who I've frequently criticized on other matters, picked up the phone and called Sidwell and they emphatically deny the claim. According to Ellis Turner, Associate head of Sidwell Friends, "Sidwell Friends security officers do not carry guns."
Obviously, the President's daughters have very high powered and armed Secret Service protection, which is a statutory requirement and the need for which is obvious to every thinking person. But the school itself does not have armed guards.