Restraining order on girlfriend or her parents... help!?

My 16 yr old son has this girlfriend that I do not approve of. Only because she is disrespectful to me and my family and she has parents that are... lets just say they have pretty liberal parenting styles. She gets whatever she wants. I am trying to discipline my son for having bad grades and ground him, but I found out that her mother is driving her to his school (they go to different schools) and spending time with him at lunch (her school is over), and she just helps her daughter find time to spend with him. How can I discipline my son if another parent doesn't respect my wishes? Can I get a restraining order against them (not that I want to go that far, but talking to them doesn't help). She feels that I am too hard on my son and that her daughter should get her way. Do I just make him stick to me like glue, he can't get off groundation cause he keeps screwing up and finding ways to get into trouble. Now I am nothing but a meanie, cause I am on his case, but I don't want to give up on him. He is a good kid in general (when he isn't with her, he has a good attitude and is happy, when he is with her, he walks on eggshells). Help!