Don't believe its a hoax, but I also can't rule out that the government and NGO's sponsor loonies like this.
How many times has the FBI been involved in mass shootings? Hell, even the 1995 Oklahoma City bombing has come under renewed scrutiny.
I put nothing past our government to get the gun control that they have been trying to get for 40+ years now. False flag or even just grooming loonies to shoot up schools to get what they want.