One possible version:
Part of the students think graffiti isn't bad as long as its content is positive because it can not only make the environment beautiful but also bring in something new for the capital of our country with a long history. Besides, they can release their pressure and feel relaxed when seeing the colorful pictures, forgetting those sad things.
However, most of the students don't think so. For one thing, Beijing hutongs are very special, which are part of traditional Chinese culture, so we should protect them instead of damaging them. For another, it is a selfish way to change public places' appearance without permission. And it hasn't been well received abroad.
In my opinion, although it is good for teenagers to draw what they like, they should do it in the proper places.