Underage drinking can be stopped with three very simple steps. Encourage parents to talk to their kids, educate the kids in school about the dangers, and enforce stricter laws. 
Parents
Parents play a key role in what their kids do. So it is their job to keep their alcohol away from their kids. Most parents don't do a great job with this because a recent survey stated that 90% of 10th graders and 75% of 8th graders say that alcohol is fairly easy or really easy for them to get. Parents should also talk to their kids about why they shouldn't drink and the dangers of drinking. Parents need to tell their kids that if they drink before their 21 that they will be punished.
School
School should have a program that teaches the students why not to use alcohol and that not everyone does. It is important to do this at school as well as home because at school the kids can talk to their friends and really see that it is not cool to drink.
Law
The government should inforce stricter laws not allowing any underage drinker to get away without punishment. The law should also be very strict with drunk driving laws. Any teen caught drunk driving should get their license taken away for a very long time.

If these few steps are followed underage drinking should stop.