Even though there are laws against underage drinking, it still happens quite frequently. which strategies would you to avoid being in a situation where underage drinking is occurring? also, how would you effectively refuse an offer of alcohol?

If I were to imagine myself to be a teen who has friends (that are underage) who drink:

I would avoid going to parties, especially large ones hosted by people that I don't know well and parties without parent supervision. 

I would also try not to associate with people who negatively influence me (for example, peers that pressure me to drink). Often those that you surround yourself with are the people you are likely to imitate. 

If someone were to offer me alcohol I would politely turn down the offer (at the same time, make sure you are confident and firm, so they don't continue to pester you).