I agree with everything that has been said here. And I am sorry to hear about this young boys situation - he clearly is hurting and needs help. What also comes to mind for me is that there may be other reasons the parents don't want to reach out for help (I think the doc mentioned this) - from what I understand, many of these types of behaviors may indicate an earlier trauma - and maybe the folks know exactly what that was and don't want it to be brought out in the open. Denial is very powerful. And fear of exposure.
I just wanted to add that the parents do not have to involve the school at this point. They can take him to a psychiatrist for evaluation privately. Of course, if the child continues to act out, the parents may have no choice since the school will take the initiative.Or the legal system will take action if they receive a complaint. That might be one way to encourage the parents to take action - by pointing out to them that if they don't get right on this, others will do it for them - and the consequences will be much, much more serious. Perhaps there are other reasonable close family that can appeal to the parents with you, as concerned and caring family members.
Plus, the child is clearly ill and needs treatment. Professional treatment, absolutely. Really, his entire future hangs in the balance.
Good luck.
Yes this is serious. This has gone beyond the concerns of your nephew and whether his behaviour is controllable, it has reached a point where the mental and physical wellbeing of others in his peer group and younger are being put at risk. There is clearly something going on in his development which is beyond the range of what the regular teenage support networks can help with - he apparently needs direct intervention by a mental health specialist, now. Whatever the cause of his behaviour, be it mental illness, misplaced anger, personality issues, the important thing now is that it is held in check. I am sorry to sound harsh but the safety of others is becoming a real factor here. The good news is that being thirteen, it may be an extreme 'phase' and ultimately entirely treatable, or he made need more long term monitoring. As yet he has not behaved in such a dangerous way as to need the intervention of social services or law enforcement, but as Dr. Greenburg states, this is a route he is heading towards.
People make a great deal of school records, but the fact is they mean little in the adult world, whereas allowing your nephew to reach the point of being considered unsafe would be questioned more. You need to make this clear to his parents. If they allow the right people to intervene now then there is more chance of this being passed through as a phase, less so if his behaviour deteriorates further. If his parents are still resistant (and yes I can understand why), maybe at some point action will need to be taken without their consent. Far far more shameful. Dr. Greenburg has put this same point much more diplomatically than me perhaps, but I am stressing this because I care, as much for your nephew as all those affected by his behaviour. It is harsh that this is falling on you to sort out, but kudos to you for taking this responsibility. You clearly love him, but love can involve doing things which initially seem hurtful, and regrettably this is the position you are in.
Confront the parents again, get your argument straight and be calm with your reasoning, prepare for their likely negative reaction. If they still resist, you may wish to raise your concerns with his teacher or headteacher, they will have more experience about how to discreetly deal with this. Just try to convince his parents to forget the whole school record concern, really, it means nothing. When as an adult have you, they, or anyone, ever been asked about your school record? There are far more serious issues at stake here.
You have every right to be concerned. The behavior that you are describing is beyond the "norm" of average expectable behavior for a 13 year-old. It sounds like you are in an especially difficult position, because you can only try to influence what your sister and brother in law decide to do or not do to best help your nephew. If you haven't already, you may want to reiterate to them that his behavior is well beyond what is expected, and if there is a serious emotional problem that is driving this behavior, that he may not be able to control doing something such as "holding a girl up against the lockers" or worse, which could have disastrous consequences. You may also want to find out if there is anything else that is impeding their willingness to take your nephew for treatment. If it is the worry about feeling ashamed that there may be "something wrong" with their son, you may want to point out that given the option between dealing with the shame if he were to be in treatment of some kind, and the potential consequences (expulsion from school, law enforcement getting involved, etc.) if he does not, it may be advisable to deal with the shame.