Lasers really can hurt your eyes because they are very bright. From a long distance you can see the spots from the small laser pointers that people use. There are lasers that are bright enough to damage or even blind your eyes. Some lasers are even powerful enough to burn your skin, or set fire to a piece of paper.
The reason lasers can do so much damage is that they produce bright light which is well concentrated into a thin beam. This thin, bright beam of light delivers a lot of energy onto a small spot when it hits something. This energy heats up the spot, which causes the damage. The same thing happens when you use a magnifying glass to focus light onto something, like your hand or an ant. The heat hurts your skin, and of course the ant gets cooked.
If you hold the laser beam steadily into someone's eye, then there is a good chance the laser beam will burn up cells on the retina (back of the eyeball), causing a permanent blind spot in his vision.
The type of laser you have is probably a small hand-held laser pointer. If you sweep the laser pointer beam quickly across someone's eye, it is probably not bright enough to do permanent damage. But this is only because the person's pupil will contract fast enough to dim the laser spot on the back of his retina. While his pupil is contracted he will be temporarily blinded, just as if he had looked straight at the sun for a moment. This is EXTREMELY ANNOYING.
One application of laser pointers is as a targeting system for a gun. A pointer spot can be mistaken as someone aiming a weapon. So anyone shining a laser pointer at a person packing heat is liable to get shot, and deservedly so.
In short, laser pointers are the most stupid, annoying things ever invented. This is a scientific fact. They don't even work that well as pointers, because any little shake of the hand makes the spot wiggle so much on the screen that the audience gets seasick. If you really need a pointer, just speak softly and carry a big stick.

Your teacher was right - it is a very bad idea to shine a laser into somebody's eyes.

Ask your teacher to show your class what happens if you shine the laser against the wall from a few meters away (make sure that none of your classmates are in the way!) Then try the same thing with an ordinary flashlight from the same distance away. What do you notice about the SIZE of the light circle which forms against the wall?

Because the light from the laser doesn't spread out like an ordinary light, this means that all the energy in the beam is focused into one very bright spot. This very bright and focused beam of light can damage your eyesight by burning the inside of your eye.

"LASER" actually stands for "Light Amplification by Stimulated Emission of Radiation". It is hard to explain the physics of how lasing works, but you can think of the laser process as being like a row of dominoesall standing on end. If you knock over the first domino, this knocks over the 2nd domino, and so on and so on,so that your whole line of dominos moves in the same direction. Here each of the dominos represents a piece of the light beam - the first piece of the light beam makes sure that the next piece stays in line and so on. By contrast, ordinary light would be like just dropping the box of dominos on the floor so that they scatter in all directions.

You might want to look in the library or on the internet for some examples of different types of lasers. Have you noticed that most laser pointers are red? (In fact blue and green laser pointers are now available, but they are MUCH more expensive). Do you have any ideas why blue lasers might be harder to make?

Laser light is very HIGH INTENSITY, so it is similiar to staring at the sun. YOU SHOULD NEVER LOOK DIRECTLY AT THE SUN THRU A TELESCOPE OR PAIR OF BINOCULARS. The intensity is so strong that it can damage your eyes and cause blindness. An analogy would be the following: imagine that instead of plugging your TV into a normal 110 volt socket at home, you hooked it up to a bolt of LIGHTNING. The poor components of your TV would be so OVERLOADED that they would BURN OUT. The same thing happens to your eyes if too much light comes in. And thats the danger of a laser...the high intensity of the light can damage the light receptors in your eyes.