Bringing your own knife to school/work because people might/have threatened you with a knife is doing it wrong; it'll just get you cut. I didn't know we'd be talking about work too, here's what worked for me: You play sissy, wait for them to walk away, then pounce and break one of their arms (you won't get away with breaking his/her neck). Be sure to explain the situation to the authorities when they arrive; Give a detailed description of the knife and where it is on the opponent's person, and after threatening you embelish that the opponent threatened to kill you the next time he/she saw you.
"Only a fool attacks head-on" ~ Vegeta.
"Only a fool attacks head-on" ~ Vegeta.