Myth. Big nasty NRA lie. Sorry, that "another big nasty NRA lie." It's been snopesed out by Talking Points Memo and the factchecker at the Washington Post.
Anyway, the idea of armed guards at a Friends' or Quaker school is a little odd.
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....TPM