Every family has a different level of silliness that is acceptable. Yet, when "clowning" behaviors detract from day to day life, cause problems with the interactions with peers, or your child is in trouble at school because of their clowning, it can reasonably be said that the "silliness" has become a troublesome behavior.
1. Give a time and place for the "clowning" show
Just as in the circus there is a time to bring out the clowns, there can be guidelines set for silliness. The root of the silliness is some sort of need for release, attention or reaction to tension (either at home or school). Giving a few minutes of silliness shows your child that there are times and places for cutting up. Depending on your child and their individual reasons for clowning and their temperament, you might try having a few minutes of goofing off allowed right before entering a situation that your child finds stressful. If your child has a hard time reining it in after being silly, use the ability to clown as a motivator or reward. ("After you finish this math worksheet, you can tell me jokes or be silly for five minutes.")
2. Expand their repertoire
Let your child see your sense of humor and help them develop ways to express themselves in a humorous fashion without being overly silly. Reach back into the recesses of your mind for the riddles, puns and knock-knock jokes that you found hilarious at their age. (This author's personal favorites from elementary school, "Why is six afraid of seven? Because seven eight nine," and "Why does an elephant paint their toenails red? To hide in the cherry tree. Have you ever seen an elephant in a cherry tree? Then it works!") This material is new to your child, and he will be thrilled to have some new material. If you have forgotten all of the childhood gems, you can find a book at the library or search for childhood jokes and riddles online.
3. Give your child the spotlight
If the motivator of the silly and clowning behavior is that your child seeks attention, give her some attention. The behavior may decrease in frequency if attention is given. Praise your child frequently for the behavior that you appreciate instead of the clowning. (I really like to hear you practice the piano.) Give your child an outlet for the attention seeking by involving him in theater, music, or dance classes so that he has a channel to get the attention that he craves in a productive manner.