There are a number of different reasons that I could use to justify why 9th graders should study this play, but really I think that anybody would benefit from studying this play, not just 9th graders. I suppose what makes it particularly appropriate for 9th graders is that their age closely matches the age of Juliet, who herself was only fourteen. Because of this, perhaps they can identify more strongly with the way in which she feels unable to make significant choices in her life because of the power her parents have over her.
At the same time, 9th graders are at that biological stage where they, if they have not already, are beginning to experience the ravages of desire and passion thanks to all the hormones flying around in their bodies and their sexual development. This play is above all else a play about passion and love and how it can completely master us. 9th graders can therefore greatly empathise with the situation that Romeo and Juliet find themselves in.