Why would Shakespeare's Romeo and Juliet be a good or bad choice of assigned reading or performance for a class of high school juniors?
The main strength of Shakespeare's Romeo and Juliet as a reading or performance piece for high school juniors has to do with the age of the protagonists and the romantic interest of the story itself. It provides many interesting themes for discussion, and teenagers can certainly relate the plot to their own lives. There are also many teaching materials available.
The main negative is that most high school juniors do not have the language skills necessary to read Elizabethan English, much less to perform it. Although at some highly selective private schools or charter schools it would be a good choice, it generally does not work out well at most public schools.
One alternative which would still have educational value (unlike doing a modern musical) would be to choose a tragedy such as "Antigone" or comedy such as "Lysistrata" which exist in modern translations. The choral nature of these dramas means that enough roles are available for everyone, especially because you can adjust the size of the chorus to fit the number of interested students, and you can find translations, such as those in the Penn Greek Drama series, which are relative easy to perform.
Romeo and Juliet would not be a very good choice for juniors. The fact that the title characters are close to their age is adumbrated by several negative factors: Shakespeare's society, as well as the fictive society in which the story takes place, had an entirely different view of teenagers; secondly, the story is worn out by modern adaptations such as West Side Story; finally, acting in a period style piece is too far removed from their skill set at that age. The function of high school play productions is to give the student a positive experience in public performance, team effort, resolute preparation, etc. The experience, then, should begin with a careful choice of material, not with some hurried, superficial selection that does not take into account the end goal of the project.