Fighting in the school hard system aims to reflect fighting in real life. By which we mean it should be short, sudden and usually happen when one side hopelessly oumatches the other.
Health leavels
Each character has health levels equal to their brawn. If a character is reduced to zero health levels then they are obviously very woozy and will want a quiet sit down a cup of tea. If somehow a character gets below zero health levels then they are knocked out. Dying is not something that usually happens. That said, if your character contrives to get shot in the face a number of times they will die. For those who need rules, if somebody manages to get down to -5 their friends would probably want to call an ambulance. At -10 a character dies in the absence of major medical attention. If they get down to -15 they are dead. There shouldn't normally be need to keep track of minus levels, though.
Losing Health leavels
If somebody takes a solid punch, or drops to the floor after a failed stage dive then they will lose a health level.
If somebody takes a dog bite to the leg, takes a fall from the first story window or gets hit by somebody using a weapon they will lose two health levels
If somebody gets hit by a car, bullet, or bitten by a shark they will lose three health levels.
If somebody takes more damage than that I will start to wonder what kind of school you went to.
Healing Health leavels
Health levels are healed at the rate of one per good night's sleep. Any health levels below the first heal at a rate of 5 per night, meaning that anybody knocked out should be coming to by the next morning, unless they're very seriously injured.
Fights
Fights are modelled using a opposed roll of brawn vs any relevant skill. Whoever loses the roll loses a health level. If the person who wins is carrying some suitable weapon, then the victim will lose two health levels.