First off, I want to point out that I disagree with the whole idea of the middle school speaking tests. Basing 10% of a student's final grade based on 2 or 3 minutes of talking while a harried teacher scribbles notes seems kinda dumb. I would much rather see that 10% replaced by a classroom participation score, or be broken up into several smaller speaking quizzes etc.
Secondly, the way a lot of schools do their speaking tests make them more a memorization test rather than a realistic gauge of a student's ability to produce authentic English. And exam where students can prepare by memorizing an answer completely defeats the purpose of the test. I had dozens of students complain to their teachers because some of the questions I asked were not included verbatim in the study sheet given out the previous week.

In any case, while singing contests, news reports etc are all really cool ideas, the fact that I have hundreds of kids to grade and some serious time constraints means that I prefer a low prep question and answer conversation format. The students know exactly what grammar they'll need to know, and will have a question sheet and book page references to study from. They also receive a copy of the grading rubric I'll be using so they know exactly what they'll be scored on.
During the test, the interview style questions themselves will be different from the examples they've memorized, hopefully resulting in answers that show their understanding of the book grammar they've learned. It's not super interesting or fun, but I feel it's the best method given the constraints I'm under.

Here's the study sheet I made for this year's upcoming speaking test, which they'll be doing in pairs:
(This is using the Dong-a-chul-pan "Middle School English 2" text book)