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.
Looks like the video just proves that if a highly trained shooter runs onto a tesing set to specifically assassinate a person, odds are they will be able to do so.
If it were a real test:
1) the shooter wouldn't know who is armed and where they are, or more precisely, the shooter WOULDN'T KNOW THERE'S ANOTHER SHOOTER IN THE ROOM AT ALL.
2) A further tewst would be done, with no defenseive shooter. How many survive in the first vs. the second. Whats not discussed is that the defensive shooter focuses all the action on them allowing others to get out.
Show the scenario where the BG just stands in the entrance and starts blasting away.