OK, this is a difficult situation, here are my thoughts - I'm not a lawyer but through my work have a lot of experience of the law and its procedures.
Facts:
We don't know what your son has done as you havn't said, but it seems likely he's made a threat.
Your son is not diagnosed as autistic, so whilst that is an interesting discussion, its not relevant.
First make contact monday morning with a local firm of solicitors and talk to their criminal law specialist, put them in the picture, they shouldn't charge for this few minutes, but you then have a solicitor ready if needs be.
It is rather common for people to say "the police are involved" when they are not, its a good threat to chuck out. If the matter has been reported it may be followed up, it might not, the police will decide.
Until they make contact assume that they are not involved. If they do get involved you may get a letter inviting your son to an informal interview, or a uniform officer may call by.
It is important to note you do not have to speak to the police at this stage, different solicitors have different views on this, these voluntary interview are used by the police as a fishing expedition to see if they can extract more info from you and if they can get enough to make a charge - never attend a voluntary interview without a solicitor present.
If you don't attend the voluntary interview the police will decide if they have enough evidence to warrant an arrest, in which case your son would be arrested and interviewed, the result is the same an interview at which a solicitor must be present in your best interests.
In respect of the girl, have no communication with her or other members of the chat group, this is grown up stuff, a letter of apology is a gift to the prosecution - if it did go all the way to court the defence is going to want to pick her integrity to bits, so you don't want to be helping her out.
With regards to the school, the fact it has been intimated this is a legal matter is a bit of a gift. If they make contact advise them that all communications must go via your solicitors.
Remember every word typed, every word spoken can and will be used in evidence AGAINST your son, so any words need to be carefully considered by a legal professional so as you don't dig yourselves in.