DS (13) in yr 8, he's a bit awkward socially (his younger brother has severe autism and DS1 has a few traits, but isn't diagnosable) but has a few friends at school he enjoys hanging out with/arguing with. He is in no way 'cool' and completely rejects wanting to fit in with whatever is popular, so I admire his authenticity and integrity!
However, this does make him a bit of a neon target for bullies. He's also relentlessly unsporty (loathes sports and P.E.), geeky and will happily spend all breaktimes, every day, in the library reading and chatting with friends.
He's recently reported that there is a particular boy who's in the same timetable block as DS1, who is singling him out for unpleasantness, mostly in P.E. or while they're waiting to go in (they line up in alphabetical order and unluckily DS1 is right in front of him), but they share two other classes. This other boy has told DS1 that he's going to get a knife and stab DS1 in the guts, that he'd better not be shit in basketball/whatever because if he's on this boy's team he'll kill him, and during the lesson will try to 'sweep' DS1's feet out from under him, trip him up in the corridor etc. 
I don't believe for a moment that the bully IS going to bring a knife to school but obviously he's made a specific threat. Even if this is all mouth and no trousers, clearly I need to make a complaint to school - but I'm very conflicted about how to, and whether it's going to make it five times worse.
I was bullied in school (with some baseless threats of violence thrown in, as well as some actual violence) and frankly my school were useless at tackling it. My life was made a misery for 4 years. (this was in the 80s) I know schools are so much better at this now, but I'm still very worried that it'll get back to DS1 and this horrible yob will just double down. 
Can school tackle this effectively without linking DS1 to the bully? He may well be doing this to other boys. (it's a state single sex school, btw)
Please tell me your stories of tackling this sort of bullying with the school.