When I was a kid, one of my friend's bigger brothers would pick on me, take my and such. I was 8 and his brother was 16 or so. I generally would get the stuff back eventually, but one day he wouldn't give me back my wiffle ball bat (does anyone even still play this?). My father walked over there with a baseball bat (aluminum) and called him out. Acted pretty menacing got my bat back, and the kid never messed with me again. When we got back home, he let me know that a fair fight is a fair fight and that I should do my best, but if I shouldn't make a fight unfair even if I am losing. He then went on to let me know that if the scales are tipped away from me (as they would be in a fight between myself and my friend's brother), that I should try to even the odds, then he started picking up large rocks to make his point. Lastly, he told me that if this didn't work and if I was in eminent danger from a much larger opponent that I should punch the crotch as hard as I could until that person relented (he also let me know that this was the only appropriate time for such an action

). We ended that afternoon working on his truck and then target practice throwing rocks. - ha
Point is, parental involvement is necessary.