The article says it was a .38, so most likely a revolver. Revolvers are extremely hard to accidentally fire if the hammer is down. It would take either a deliberate long pull of the double action trigger or a deliberate cocking of the hammer to fire in single action mode. Firing that gun wasn't an "accident", he was dry firing it and hadn't confirmed that it was empty.
The ricochet was off of a desk and into the students arm, so the gun was pointed in the direction of the students seating area, not in a safe direction.
At 73 years of age, the instructor has probably demonstrated how a revolver works hundreds or thousands of times. He got complacent and didn't follow all the safety precautions he was trying to teach.
I feel for him a bit. This is what happens when you make a career out of handing something as potentially deadly as a firearm. Most of us only risk a chewing out if we screw up at work. The penalty for failure in this instance was a bullet to a students arm and the end of a career.
The instructor is lucky he isn't in jail, especially with our current culture of demanding someone be held criminally responsible for every "accident."