Need to give group leader a chance to handle it

If you go straight to the boss, you'll run the risk of offending the group leader.

The whole problem was created by your over-reaction. After you discussed with the group leader, you should ask if the group leader would do something about the girl. Managing people is not your job. You can discuss the issue with the girl privately, but you can't judge her in front of the group leader. By doing that, you already offended the group leader.

Remember, you don't get to play judge even when you are right.