I'm very concerned because we have talked to him about the dangers of these items and how this is not the way to impress his friends. How do I get him to stop this dangerous behavior?
Sit down with your son when you are both calm and explain again why he is not allowed to have lighters and knives. Let him know that since he has shown you that he is continuing to take things to school, you will be checking his bookbag and pockets each day before he leaves. You may want to purchase a clear plastic bookbag so you know what he is taking with him. Make sure that the teacher knows about these efforts and ask her to keep an eye on your son in case he does take something that could hurt someone.
Also talk with the school counselor. She may be able to give your son some individual time or include him in a small group. You may want to get some additional help for your son outside the school with regard to his stealing and misbehaving. The counselor or your pediatrician can refer you to a therapist in your community.