Looking at aerial views and some crude sketches of the school plan I have seen on-line, it strikes me that an armed assault through the classroom door wasn't even necessary. This school more than likely had windows, and it's a one-story building. Set up a sniper team where they can get a view of the bad actor. Three-person team. When the designated shooter has a shot, two others fire (shotguns?) to break the glass, followed by the designated shooter taking out the bad actor.
If the windows are small and the bad actor is pacing around -- set up multiple teams, from different vantage points.
If the SEALS could take out three Somali pirates shooting at a bobbing lifeboat from the deck of a ship at sea, a police sniper should be able to take out a teen-age punk at a distance of probably 50 yards or less.