Westminster Cathedral has brought back the medieval tradition of the boy bishop. Check out Msgr. Langham's blog for the article and pictures.
The medieval practice of the Boy Bishop varied from Cathedral to Cathedral, some celebrating it on the feast of St Nicholas, others on the Holy Innocents. In each Cathedral, however, the (adult) Bishop would vacate his seat for the fully attired chorister to take over from him. The chosen boy was surrounded with magnificent ceremonial: at York, the Boy Bishop was taken in procession around the Diocese visiting churches and monasteries, while at Gloucester he received lavish presents from the nobility. The Boy Bishop would always preach a sermon, which attracted huge crowds. The practice was banned at the Reformation, but has in recent years been revived at Gloucester and Hereford Cathedral. Here at Westminster, with our strong sense of continuity with English medieval Catholicism, it is good to see an ancient tradition reborn with such enthusiasm.
I have to say that I do not agree that this is a practice that should be revived. I think it is something that should have never began and was rightly suppressed. Children should not preach sermons at Mass, but much less children who have not been ordained. This entire ceremony looks too much like a farce. This is just one article of proof that liturgical abuse is not a modern phenomenon.
A commentor has informed me that the Boy Bishop's sermon is given after Communion. Therefore, it is more properly a reflection and not a sermon in the proper liturgical sense. It is liturgically correct.
Here are a few links with more information on the tradition.
Full Homely Divinity
St. Nicholas Center
Saint Nicholas Kids
Just to clarify, I also think "mayor for a day" and "principal for a day" are stupid ideas.