All a prospective shooter has to do is keep the gun out of sight and it's the same result.
Yes, the first classroom. But perhaps the teacher, after hearing gunshots down the hall, would be inclined not to open the door for a stranger.
RE: CHS and Salmen, I believe every high school in St. Tammany has had a 'resource officer' on campus for several years. I know the one at CHS in Covington PD; Mandeville High is Mandeville PD.