You have reason to be concerned. The 'normal' situation is for children to display behavioral difficulties (even normal spectrum behavior problems) at home, while behaving in perfectly good ways in out-of-home settings. Your son is doing the opposite and this invites professional evaluation. Now, this is not meant to imply that he is displaying the signs of a serious problem, but you should definitely arrange an evaluation.
My daughter is exactly the same way. She is sweet, kind, and respectful at home, but at school she loses focus and does not seem to have the same attitude. I wish Dr. Kevin Kennedy would expand on his answer. Although my daughter's behavior is not too terribile, I'd like to have more of an idea of the exact cause for consern.
I am ateacher and I have known some cases of children who behave well at home and not good in school. These children are in the category of gifted and talented. But please do not take this comment and use it as an excuse that everything is fine for the behaviour. You should research this by talking to his teacher. There are also lots of sights and checklists you can use since you said he is bright. It is very serious though to run away from school as this puts his safety at risk. I would in the mean time, discipline him at home in ways you feel are appropriate based on his behavior at school. Don't feel that you can't do this just because the behavior happens outside of the home. Also, try talking to the teacher and offer what you do at home-this could help at school.