Your physics teacher was wrong. I have read of several people being killed in Miami by bullets that were fired straight up in the air. It's a custom in the Cuban community to celebrate holidays and such by firing guns into the air, and Miami radio and TV warn people to get under a bridge or some other substantial covering before midnight on New Year's Eve.
The terminal velocity of something as small as a bullet (e.g., 38 special=158 grains) is not lethal.
However, when people randomly fire a bullet into the air they do not fire it directly perpendicular to the earth's surface. Especially not a bunch of drunken cubans and mexicans on New Year's. Such a random shot will be fired at an angle which makes the bullet lethal.