Children who have a hard time speaking in public or even making friends at school are sometimes considered shy.But being extremely shy to the point where at home they can speak but at school they literally freeze up could be something more serious -- an anxiety disorder called selective mutism.Anxiety disorders in general are the most common disorder in children, doctors said. And even though selective mutism was first identified in the early '80s, a lot of people still don't know what it is. Many people just see that their kids are shy, when in fact, they may need help.At home, Kayla and Audrey Villa are like many other sisters -- rambunctious and loud -- but at school, they wouldn't talk even when a teacher asked a question."She just didn't speak or didn't interact with any of other kids," said Carol Villa, the girls' mother."It's very much an extreme shyness. We think it's an anxiety disorder," said Dr. Marianne Wamboldt, with Children's Hospital.Doctors said selective mutism is when children such as Kayla and Audrey get so much anxiety of having to speak to strangers that it becomes painful for them."They would never volunteer. And if asked, they get so upset by it, they cannot respond to the teacher or read a passage from a book," Wamboldt said.Doctors said selective mutism is treatable. In fact, one way to open kids up is with art."We utilize art and music to slowly draw them out and help them feel more comfortable and also see that other kids might have similar problems," Wamboldt said.After therapy, Kayla is already showing a difference."I'm happy that I can at least go up to someone and talk to them," she said.Audrey is still going through therapy. Despite some progress, selective mutism takes time to treat."Most of these kids with treatment end up doing very well," said Carol Villa.