While I agree that some degree of rough play is normal among peers - my DD and her male friends - she was the only female body, but was/is tougher and more aggressive than many boys - did this sort of thing interminably, especially when they thought adults weren't watching; this isn't among peers, this is between a gang of older boys and a younger, more vulnerable one. Frankly, this is "Lord of the Flies" behavior - and even if in some cultures it's seen as OK, I am of the option that it is ripe for deselection.
DD having been in judo in middle school, I'd say that the kids either need to be told that you're going to tell their sensei/instructor, or (as her teachers told us) need to be told that THEY need to tell their instructors before you do. Martial arts training, in every dojo I've ever encountered (even those in film, short of the Cobra Kais in The Karate Kid) this would result in a lot of boring, physical labor (the judo dojo's parking lot was swept with hand brooms and the floors polished with rags) or some other form of mindful punishment.
Your DS sounds like exactly the kind of kid I wanted to raise - kind, quick to act for others when possible, and mindful of his duty to protect those that can't protect themselves. My DD learned this early, too, and I'd like to think she's a pretty awesome person.