A Fontana High School student grabbed a hammer in woodshop and clubbed another teen, sending him to the hospital in serious condition with a head wound.
The attack apparently involved a dispute over a girl.
Police Sergeant Jeff Decker says a 17-year-old boy sneaked into the shop class Tuesday and hit the victim, also 17, once with a hammer. Earlier in the day, the attacker asked the woodshop teacher what the penalties were for murder.
Police arrested the teen and he was booked for investigation of attempted murder.
The wounded student remains hospitalized in serious condition.
Principal Sergio Flores says the attacker will be expelled.