Well, at least you aren't taking DARE (which either has no effect on later drug use, or actually CAUSES more kids to eventually use drugs). But Science class is NOT the right class to be teaching about peer pressure and drugs. Health class is somewhat better.
Anyway, marijuana's scientific name is Cannabis Sativa. Some other common names for it are weed, pot, reefer, green, dope (though this can also be used to refer to heroin), smoke, chronic (this can also refer to a specific variety of marijuana on the West Coast and also can refer to marijuana mixed with cocaine), hash (though properly hash or hashish are made the THC crystals from the marijuana plant), Mary Jane, and grass.
One can either smoke marijuana, eat it in foods, or vaporize it. A vaporizer is a device that heats the marijuana but doesn't burn it, causing the marijuana to release THC but without any damaging smoke. Cancer patients who use marijuana usually vaporize it.
The only actual dangers of marijuana come from it being illegal. Though smoking marijuana can cause some slight lung and throat damage, it has never been linked to any smoking related illnesses like tobacco has. Marijuana is not physically addictive. It is physically impossible to overdose from marijuana, in the history of humanity NO ONE has ever died from marijuana!
If you want to be honest in your little song, stress that marijuana is illegal, and so you can be fined or put in jail for using it. Those are the only dangers of marijuana!