As you've seen, it's difficult for some people to admit they've done something wrong. Be sure to model this behavior for your son by admitting when you've made a mistake, and follow the positive system I've just described by rewarding him with your attention when he admits to something he's done.
Since your son's behaviors appear to have been going on for awhile, you may want to get some additional help for him. Talk with the school counselor; he may be able to give your son some individual time or include him in a small group on good behavior. If you decide to get assistance for him outside the school, the school counselor or your pediatrician can refer you to a therapist in your community.