Here's a hypothedical situation :
Let's say you have grounded your teenage son from leaving the house or using the phone for a month. He has since taken your phone without permission. But you let that go. Just to avoid a fight. Than next thing you know he is across the street. You go to get him, you find his arm around a girl. The same girl he was with when he orginally got grounded. Remember : This boy will argue back, deny any wrong doing and play stupid about being grounded. What do you do? Do you ignore the girl? Ground him longer? Or forbid the girl from coming over?