David,
I think you hit the nail on the head in suggesting that at least some of the playground aggression comes from the pressures of high-stakes testing, etc. I also think that, within the framework of what we are currently doing, your suggestion about supervision makes sense.

However, I would like to encourage people to look deeper and think about more fundamental changes in how we treat our children. I think much of the aggression we see is an artifact of (a) the general competitive nature of schooling and (b) age segregation.

Age segregation is a modern artifact of schooling. During all of history before modern times, especially during hunter-gatherer history, children played in age-mixed groups. I have been observing age-mixed play for years--not as a supervisor but as a researcher watching unobtrusively--and I have yet to see any serious aggression. The children I have been observing are students--age four on through teenagers-- at a democratically organized non-graded school. Sometimes teasing occurs, but when it does some older kid (who may be just 7 or 8--younger than your fifth graders) usually steps in, quite naturally, and tells the teaser to knock it off.

I have also observed, in a variety of contexts, that kids of all ages who are supervised are more aggressive than those who aren't. They are also more likely to behave in other irresponsible and sometimes dangerous ways when supervised than when not supervised. It is as if they give up their own personal responsibility when they think an adult is in charge--responsibility that they manifest when no adult is around.

I'm dealing with issues such as these in a new blog that has some connections with yours. It's at http://blogs.psychologytoday.com/blog/freedom-learn.

Thanks.