Pencil stabbing? Over the use of words? no offence but you need to not be so stressful, go relax and don't take stuff so uptight. That's fucking scary man, if I ever saw someone stab someone with a pencil after punching someone, I wouldn't want to be near you. This was not a fight, you got angered by the words he said, it's fucking words, they mean shit all. You punching him and then stabbing him without him punching back or anything is real violent. Lay off the stabbing games bro.
But OT: Tell your parents he was saying mean words, then you punched him and stabbed him. There's honestly no other way to tell them any better since the school teacher and other students probably will/already have talked to the principal and gave them the full story. Either way your parents will know that you stab people after they say a few mean words that couldn't mean a thing coming from a teenage guy at your school.