Police believe Jason was trying to stop an argument between a partygoer who had asked three gatecrashers to leave when he was slapped twice to the head by one member of the trio who was holding a long, sharp object, believed to be a screwdriver.
The 17-year-old was then punched by a second gatecrasher before the trio fled.
Friends have told police Jason seemed dazed and had a trickle of blood coming from his forehead so they put a bandaid on the wound.
It was only later when he began to lose consciousness that it became clear that the assault was more serious.