Is it a bad idea to write about a demonstration?

Answered

This sounds like a strong idea that nevertheless has some potential pitfalls. To start with your last comment, talking about 'politics' in college essays isn't impossible, but you should observe a couple of ground rules if you want to do it. Basically, try to avoid making broad sweeping claims or comments about the world as a whole, big countries or systems, or large groups of people. Try to focus very specifically on yourself, and how your experiences have shaped who you are.

That leads me into the second possible pitfall with this essay, where it becomes more about the issue you're talking about than yourself. I've seen students write really effective essays in the past about political topics, but I've also seen students write essays that ended up being more about those topics than about themselves—and that's the biggest issue you might run into. If you end up with 650 words that are primarily about the movement you're discussing and why it's important, that actually might not tell us all that much about you, your beliefs, and your goals for the future. Make sure the focus in this kind of essay remains consistently on you and doesn't just become an informational or persuasive essay about your country, its politics, or the movement you're discussing.

Separately, it doesn't really matter what you're applying into—this could still be a good essay. For a lot of schools, you'll need to write a separate essay anyway about the things that you're interested in, and the Common App essay is mean usually to be a less academic, more personalizing and humanizing piece. So no matter what you're studying, this should be a fine topic to choose.