Does your child have inappropriate friends? You know the ones I mean. They are not the ones you would pick for your child. But be fair. You must ask yourself if these new friends really inappropriate or do they simply play havoc with your comfort level.
You hate the way your teen is dressing, you dislike the slang language he is using, etc. Remember you were once the same age. The way you dressed and the slang you used may seem harmless in comparison, but it was probably just as offensive your parents. If after an honest evaluation, you still have concerns you should take steps immediately to help your teen remove himself from this negative group as quickly as possible. Especially if you believe is a bullying group of kids. However, it may be difficult because, (1) peer pressure can be tremendous and (2) it is normal to want to fit in.
You may notice that your teen has stopped seeing his long-time or "parent approved" friends or makes disparaging remarks about them. If your teen no longer shares those "Hey Mom, guess what" stories and has become withdrawn or secretive, ask what his friend David is doing these days or something similar. If the answer is "I don't see him anymore." try to find out why without showing how disappointed you are, or saying, "Why?, David is such a nice boy!" Give the new friends a chance. Make sure your teen knows that it is fine for him to invite them to his home. They should feel welcome and not scrutinized. You will not be able to get to know them if you have already made a decision that they are just no good. Keep an open mind and talk to them in a non-threatening or judgmental way. Don't bombard them with questions. When teens feel comfortable around an adult they are more likely to open up.
A couple of things to watch for when your child is with his friends, is he a true part of the group or conversation? He should be. Watch for body language and tone, this will help you to know if there is real comradery or something less healthy going on. Hang around (not in their faces) and listen to the conversation, is your kid being teased or bullied? Is he the one doing the teasing and bullying? If he is the bully then you have another problem.
Your teen may start missing curfew, if it is just 10 or 15 minutes, he may be testing you to see how far this can be pushed. If this is becoming a pattern, explain the rules again and let the teen know why his present behavior is unacceptable and the consequences of continuing it.
If, as your child became older, you thought it was a good idea to become more of a pal than a parent, return to being a parent. Your teen needs a parent that he can look to for advice and guidance. Sometimes kids want to be told "No." There is no better excuse than, "My mom or dad said I can't go." It absolves them. They are still cool.
Good parenting should be support, not iron control. With your help your teen can more effectively move away from inappropriate friends and perhaps change his own negative habits. One more thing, if you have a "gut" feeling, go with it, no one can protect your babies the way you can.