Talk to the student in private. In a calm manner, ask him why he has been disrupting the class. Let him know that you don't appreciate him interrupting your lectures and distracting the other students. Tell him that you want him to start behaving better in class.
Discourage her unruly behavior in class with nonverbal cues. For example, you can stop what you're doing and make eye contact with her. Discuss these nonverbal cues with her beforehand, so she knows she's acting up if you give her a stare.
Ask the other students not to respond to his behavior when he's acting up. Let them know that laughing at his antics only encourages him to keep disrupting the class.
Pay attention to her. If she receives a good grade on a test or answers a question correctly during a class discussion, respond positively. For example, say, "I'm very proud of you for getting an A on your math test. You can do anything you put your mind to."
Discipline him if he continues to act up in class. Have him stay after school one day, or have a talk with his parents. By doing this, you're letting him know that you won't tolerate his disruptive behavior in class.