What is School problems (higher absence and poor or failing grades). Social problems (fighting and lack of participation in youth activities). Legal problems (arrest for driving or physically hurting someone while drunk). Physical problems (hangovers or illnesses). Unwanted, unplanned, and unprotected sexual activity. Disruption of normal growth and sexual development. Physical and sexual assault. Higher risk for suicide and homicide. Alcohol-related car crashes and other unintentional injuries, such as burns, falls, and drowning. Memory problems. Abuse of other drugs. Changes in brain development that may have life-long effects. Death from alcohol poisoning.
http://www.cdc.gov/alcohol/fact-sheets/underage-drinking.htm