Most teachers will say that in hopes of scarring you so you won't smoke weed. I honostly beilive that weed is a gate way drug and can cause people to move up to more "daring" drugs. Yes, alchohol is more dangerous than weed but just think: our lungs are ment to breath nothing but oxygen, how could breathing in anything other than oxygen be "ok" to do. Smoking weed is just as bad for your lungs as smoking tobacco is. It causes lung damage to smoke ANYTHING.
No, weed does not "kill brain cells" but it does slow down chemicals in your brain. This is what makes you feel "high". You are less coherent and aware of the world around you. This makes it dangerous to drive or function outside of your home. Everything seems "slowed" because of the effects smoking weed has on your brain. So, for example, if you're driving in a car while "high" and you see a street light turn red, your brain will not register that you need to stop as fast as it would if you weren't "high" so you might not press on the brake until you've already hit the car infront of you.
Keep in mind that I'm entitled to what I beilive, and YOU are, too. If you wish to smoke weed, that's your choice and I have no right to judge you as a person for that.