It’s cross dressing wrong? If yes, why? And if yes, is it ok to do only for a play. I tend to think it is not right. My child’s school has the middle school boys do a skit where they dress up like girls in very pretty dresses. I don’t like it, but it has practically become a tradition. My son actually said he is looking forward to getting to do that next year, and it bothers me. What do you think?
Speaking from a theater perspective, it’s actually a very old tradition for women or men to play opposite-sex parts. For a very long time, women didn’t act on stage at all–at least, in reputable theaters, if you get my drift.

Shakespeare, for example, would have been acted by all-male casts at the time, with not an eye batted about it, either. Ancient Greek plays were the same way: the classics of Sophocles and his peers, and the tales of ancient Greece and Rome would have been acted by exclusively male casts, and there were plenty of female roles in those plays.
For some time in opera and in Baroque vocal works, many female parts were sung by castrati–men who were castrated (yes, I’m serious–look it up!) as boys in order to maintain ranges in the soprano/alto vocal registers so that they could sing those roles, because again, “nice” women didn’t sing on stage. The Church got a bit huffy about that practice after a while, as you might imagine, and the castrati eventually died out, which meant that women appearing on stage became an accepted practice. This led, however, to the existence of the term “trouser roles”: male character roles with a female (generally alto) vocal range, and therefore performed almost exclusively now by women. (There is a very rare male vocal type known as the countertenor, whose singing voice is naturally in the soprano/alto range, but I digress.)
Likewise, in boarding schools/other single-sex schools which have any kind of arts program, some kids have to play the part of someone of the opposite sex. When I was a girl, some friends of mine and my sister and I would act out fairy-tale plays for our families. Since we were all girls, a girl always had to play the prince. Usually me, because I was the oldest and therefore the tallest, and I can assure you that I had no ideas then or now about being transgender or a lesbian from the experience.
In short, acting a part of the opposite sex is generally accepted as okay, and no implications about the people involved. As to your specific question, though, it really depends on what the point of the skit is. It is “oh how stupid, boys wearing girls’ clothes?” Is it to mock women or girls, or, for that matter, men and boys? Is it a passage from a classical Greek play in modern costumes? Does doing this mean that those boys are going to be teased unmercifully for the next six years? Could you give a bit more context?