The decision to have your son diagnosed is a very personal one, but as a parent of a child with autism and two children with ADHD, I faced these choices too. I fought to have the diagnosed removed from my daughters record only to find out when she got to college that with the diagnosis she would have had much needed help and support. So, the diagnosis and "labels" have actually helped, also, the schools are very cautious to keep this confidential, especially when its something slight. So, talk about it with others in this situation, ask someone trustworthy in the school about services, perhaps you can can have help without this label. What kind of person is the principal or counserlors? A word of caution is that usually, their main concern relates to nickels and dimes. Don't think that their main concern is always for your child, this is teacher to teacher, principal to principal, school to school...
As far as grounding permanently, this never works. Look for a way for him to earn privilidges that he wants back..., What does he want? What would he work for? Deos he have a girl friend where phone and computer time (with supervision) would work? what is he trying to tell you, by his behavior, it sounds like he is acting out. You may need the help of a professional, at this age there is still time, but you have to act soon. If he was diagnosed, the school would be compelled to help you set in place a behavioral plan, after a functional behavioral assessment. I hope that this doesn't sound like mumble jumble you may have to contact help from an advocate (usually free) to find this kind of help.
The best approach is to try to team up with the school if this is an option. Again, It depends on the school. Punitive, measures like long term groundings rarely work. At 13 our childrens brains are still not fully developed and they see this as a personal attack on them, which fuels and supports their vision that they need to defend themselves and continue with the "bad" behavior. Again, a method to help him earn things that he wants and figuring out why he is doing this can help. The first thing to do is to "define the problem" in order to start seeking solutions, with your sons input. And yes, when they feel attacked children seek those whom they seem as being in a similar situation, which ususlly exacerbates the problem.
I know that this is a page full, I hope it makes sense,