Being told to die

Perhaps it might be a good idea to make a record of these incidents - times, dates, places and who is involved - and give copies to your teacher, head teacher, and the police - in my experience boys who get away with physical stuff once or twice become really dangerous. When you show the record to a teacher or headmaster ask them to sign it to say that they have seen it - so if anything further happens you have evidence of their inaction.
Do not slap the boys - you might get badly hurt or worse - but if you are grabbed and they try to drag you away assume that you are in mortal danger and bite, scream, kick, gouge - anything to escape.