The video "proves" the same thing we preach on here all the time : Training and Practice are crucial tools. But, the exercise was a bit rigged, the BG's knew there was an armed student, and, who he was. I don't think the hoods disguise the students to the point that a trained officer could not pick out the armed target and neutralize him before he could deploy the weapon.
The gunman didn't need to pick out the armed student at all. The setup was that the armed student got delayed while receiving the gun, and then they enter the class after everybody else is already seated. The armed student never had a choice in seating positions so the gunman always knew exactly where the armed student was.