Why do teenagers under the drinking age have to go to parties that involve alcohol?
Just as the teacher before me stated... THEY DON'T.
But, if you are asking this question as a preview to reading the book Speak, you must consider a few things.
There are parties in high school that don't involve alcohol. These are the best.
Those that do, are the kind that happen in Speak. You may be about to read Halse-Anderson's book, and her main character experiences a party between her 8th grade and 9th grade years that defines her for the rest of 9th grade socially... at least she feels that way.
Generally, the perception is that drinking is 'cool'. Losing control feels good for the time being to some and the problem is that those who are new to alcohol have a very low tolerance. So, they could drink enough to kill themselves the first time and not even know where that line is. This would be the case for an 8th grader trying to hang with high schoolers. They would want to get involved and see someone older maybe drinking a lot, but as a new-comer, drinking like 2 beers could seriously impair decision making.
As advice, I would say DON'T EVER START.
As a preview for looking at Speak, I would say, think about how self-esteem affects decisions. This girl was affected by that. Watch how it plays out for her.
They don't "have" to go. They choose to go to bolster their status in the eyes of peers, or to bolster a sagging self esteem. Once at the party, if others notice they aren't drinking and razz them about it, they may start drinking to keep their status and self-esteem intact. Unfortunately, alcohol loosens inhibitions, and unfortunate events may follow. The need for a peer group in teens is a major influence on life choices.