What are the signs of alcohol abuse in a teen?

Underage drinking. Technically, ingestion of any illegal substance constitutes abuse. Teens may not legally drink alcohol. Therefore a teen who is drinking alcohol is suffering from alcohol abuse.
Various. Safe use is far far less for a teen especially younger teens. Abuse has significant social, health or other impact on functioning. Frequent binges, missing school, withdrawal symptoms, intoxication, mood changes are all trouble. See http://cpancf.Com/articles_files/talking%20to%20your%20adolescent%20who%20drinks.Mht for more help on talking to your teen abut drinking.
Functioning. Impairment of functioning in all major life areas are signs of substance abuse. These include problems in the family, with friends, with work and school. Also, a change in a person from their usual levels of functioning may point to a substance abuse problem.