Art Haycox Elementary School in Oxnard locked its campus when a teen showed up at the school covered in blood.
Principal Julianne Pena said the boy had blood on his face and shirt and appeared to be running from other boys.
Police thought the teen had been stabbed by the people chasing him, but they now say he cut his thumb with his own knife during a fall.
Pena believes the teens had been waiting for a bus to an alternative high school.
She said staffers at the school have called the police before because of teens causing trouble at their school bus stop. Pena said one morning they could smell pot coming from the bus stop.