Sure it might be the wrong way to unload a weapon in a crowded classroom but its the fast way.
Not sure how he was able to carry on with the class after A): Shooting himself in the leg and B): With any semblance of authority on firearms.
I think I might know how it happened. He cleared the cartridge in the chamber first by racking the slide (hence, re-loading the gun with the next cartridge) then he dropped the magazine out of the gun. Instead of taking out the magazine first then clearing the chamber.