As we've just confirmed, Romeo and Juliet remains most students' introduction to Shakespeare (at least in the US, assuming approximately a 9th grade / 14yr old introduction). The problem, as many have also pointed out, is that Romeo and Juliet has got a crazy amount of sex references in it, and typically a high school teacher (again, at least in the US) is severely constrained in exactly how far he or she can go in explaining these things. Lastly there's also the question of whether Romeo and Juliet is the best example of Shakespeare's work to start with. Maybe being a teenager has changed since I was a teenager, but the thought of re-enacting the balcony scene with some random girl from class was always to be met with "Oh god no not that I hate that don't make me do that" feelings.
So then, here's the question : Should it be changed? Assume the following : You must introduce Shakespeare to United States school children in a way that could be accepted as national curriculum (i.e, we can't talk about special case "let them pick their own" situations, we need to actually pick one). When do you introduce it (roughly what age), and what play do you start with? Why? Do you teach it as history, as literature, or as drama? I fully expect "a combination of all three" answers, so let me rephrase that - assuming that Drama, Literature and History are different departments taught by different teachers, who will be teaching Shakespeare? To set a baseline let us also assume that the students would be required to read the play, have some degree of homework associated with the play, and be able to pass some form of test demonstrating their knowledge of the play. This is primarily to rule out the "I took my kids to see The Tempest when they were 3 years old!" argument. I will not try to argue that my kids "know Shakespeare" until they have experienced it to at least this level.
Keep in mind the realities of the situation - there will most likely be a bell-curve of students, some of whom excel, some of whom just can't seem to get it, and a whole bunch in the middle who may or may not care at all. Any play that has any level of performance involved should take into consideration roles for both boys and girls (or at least, have a plan for how to deal with this).
If you want to defend Romeo and Juliet as still the best choice, feel free.