Illicit substances have a powerful appeal to young people. Alcohol is ubiquitous in the media, and very smoothly marketed. For people who have not yet developed the social-analytical powers needed to resist such advertising, the idea of such things as 'cool' and generally appealing quickly becomes embedded. By the time they get to actually try alcohol, they are more than eager to do so - primed by the media and social conditioning they have experienced.
It Lends Escape
Everyone goes through a degree of self-exploration during their adolescence. However, for some this is more bewildering than others. Drugs and alcohol provide an escape from the turbulence of puberty which proves perilously attractive for some.
It Lends Confidence
Teens who lack self-confidence may use alcohol or drugs as a way to become the confident, noticeable person that they want to be. High school can be a vicious environment for people stuck within their shell, so anything which helps someone out of that shell is yearned for - no matter how damaging it may ultimately prove.
Part of the process of becoming your own person involves testing the social boundaries and setting yourself against the authorities which have always ruled you. Drugs and alcohol are often seen as a great way of gaining this rebellious control.
Many teens are stressed and depressed. Alcohol and drugs provide temporary relief from this, which may motivate anxious teens to try them.
For more on this, see this article.