Oh, Cris, what a bummer. We've been there (well, not quite beating the other kids up, but certainly being too physical/fighting, and being very defiant with the teachers), and it's not fun for anyone.
We also used a hard line here -- there were no exceptions, and punishment was swift and (probably overly) harsh. Like your ds, ds6 doesn't really miss video games or TV.
So we started with earlier bedtimes. On Day 1 of bad behavior, it was a half-hour early with no reading, lights out immediately. On Day 2, it was a full hour early -- that makes bedtime 7 pm, and when we don't get home until 5:30, that's *early*. And on Day 3, he had to sit on the couch and do *nothing* from the minute he got home until his massively early bedtime at 6:30. No talking, no playing, no reading, no video games, no TV, just *sitting*. It was pure torture for my very talkative, inquisitive kid. One night of that and he shaped up quick.
I think it worked because it was fairly immediate -- as soon as we heard the report, that was it. It was short-term -- only one night -- but had the potential to snowball into a fairly horrible punishment. But if, after a bad day, he got a good report, he was immediately rewarded that night with a regular bedtime, reading time, and sometimes even an extra treat. So we'd reinforce the good and really go after those fighting behaviors.
It was a punishment for me as well; I work all day and it was a bummer to put him in bed at 7! But it was the best, most effective thing we've tried. We also had a daily journal for reports from his teacher.
We'd remind him in the morning -- what's going to happen if you get a negative report? He'd repeat back the consequence, so we knew he understood. I think this reminder helped just before he left, so he'd have it fresh in his mind. We'd also remind him that he may not be able to control the *other's* kids' behavior, but that we expected him to control his *own* behavior. We did some role playing: "What will you do if Sam comes up and tries to fight?" I think that had some effect too, as he knew he had some rehearsed phrases in his repertoire to deal with bad situations.
And there is, of course, the challenge at school. We seen a huge decrease in physically aggressive behavior with appropriate placement. When his mind is active, apparently, he doesn't feel the need to be so physically active. He's still very chatty, but he's not being defiant and aggressive at school anymore. He's a much, much happier kid when he's really working at school.
Good luck. Keep us posted! I really, really hope the gifted school works out for your ds next year, it's been a lifesaver for ours.
