What would you do???

For the past 3 or so weeks i have noticed a bunch of middle school kids getting off the buss in front of my house constantly beating up on this one other kid. Its like 3 even 4 against this one little kid. This one time i saw the kid in tears and wated to just to out there and beat the sh** out of those kids. ONE problem, Im 19. I know its not worth going to jail over. I yelled at them once, but that only provoked me more. I know boys will be boys but this is just wrong. I want to do something but im not sure what. Any advise would be cool. THANKS!!