Hmm, as another 15 year old (who should be doing homework right now), let me chime in here...
I don't think there's anything wrong with the occaisional drink or two (as kelly has stated), and in fact there's been a slew of recent medical research linking a glass or two of wine a day to reduced risk of heart disease and dementia.
On the other hand, underage drinking has been shown to affect brain development and a lot more stuff along that lines - generally, alcohol tends to do the most damage while your brain is still developing; after that, its effects tend to diminish, a lot.
Combined with the fact that I don't really like the taste of alcohol...
Drugs, apart from pot but including tobacco, are fairly different. I've seen enough research to show that they're quite addictive and quite damaging (and I'm not talking D.A.R.E. here) that there's a reason they're classified as Group/Section 1 - substances without any medical use that are harmful enough to be outlawed.
Pot, on the otherhand, isn't physically addictive (though psychological addiction is still possible) and seems to do less lasting damage than smoking, so I'm not quite sure why it's in the same class, as say, heroin. (for the record, I haven't tried pot; I don't need any worse of a memory than I already have)
Please correct me if I'm wrong in any of the above...