Other - Run. I'm not getting stabbed in the stomach, thanks! o.O Even if they're younger, you'll never know if they have a knife.
Otherwise, if they're definatly unarmed and try to attack me, I'll probably fight back, punch them in the face or something, anything. Heh heh when I was in year 7, two Year 10s were kicking the back of my friends shins as they walked. My friends went on ahead but one of the year 10 stopped to tie their shoelace so I kicked their leg, pretty hard...she got up and looked around at me so I backed her into a corner and she started to say that it was her friend, not her. I suddenly realised what I was doing and ran off to my next lesson. It may sound like nothing to anyone, but it was to me. I'm a pretty quiet person, so people don't think I'd do anything like that whe I could've walked away heh.