I would not advise yelling at them. This makes it seem that you're even more out of control and lets them know that they have successfully pushed your buttons.

The problem is that the kids with whom you are not directly working are idle. Give them a job or some sort of responsibility when you cannot directly supervise them. Have them sort, alphabetize and file music. Organize, decorate or rearrange the room. Have a music composition contest and let them work on their own tunes while you're working privately with a student, then set aside a day for the competition. (The winner should get some special recognition; you could even make the competition an annual or bi-annual event.) Create and bring in music-related skill or trivia games for them to play, require them to do some kind of productive research (but NOT as punishment) and then count the assignments toward their grades.

Reward them when their behavior is appropriate. Have sessions outside, bring snacks to class, order and let them eat pizza, etc.

If they continue to be disruptive, issue detentions and/or contact their parents. Keeping the parents informed is very important because most parents actually do care. Call or email them with an update every few days if you have to in the beginning. If you do this, you won't have to do it for long. The kids will get the message that you're serious, you will do whatever it takes to maintain order, and they will stop calling your bluff.