Do you have an assistant? Are YOU staying calm or are you getting nervous and are the kids picking up on that? What is your routine, how do you start the circle? Who puts the chairs in the circle you or the kids? Are they coming from free play? Do they put the toys away first? How do you get them to put the toys away?
Maybe have everyone take a seat at the table and and shut the lights and turn on the tape for one song. Take turns having someone choose which song to play, maybe the child who behaved the best during circle time or a child that was chosen at circle time the day before. When the song is over turn on the lights and have everyone take their chairs and make the circle and then get started. At the end of circle time tell them how proud you are of them for sitting so nicely and being so eager to learn and pick the child who will choose the next days get ready for circle time song.
Sometimes routine is a key for children, they need to have the structure and they need to know what to expect.