To my shame, once or twice. Not because I liked it- I hated it- but because it was that or have them turn against me. I was so used to being the target it was a massive relief when it wasn't me for a change. I was bullied rather seriously- going far beyond "teasing"- and when it was someone else I said nothing because for a while I was secure. Once or twice, I even participated slightly. Luckily, the targeted boy in question knew why I was doing that, and I don't think he ever held it against me. Still, I feel guilty about it to this day (it wasn't anything serious I did, but still, I wish I'd been a little braver)
Really, my memories of age 11-16 are horrible. It's terrible the way children treat each other. In my community it was particularly bad. I don't actually know if it's only here, but from what I've heard it's everywhere. Our societies are so competitive and aggressive. Children are taught that if they kick someone else down and stand on their back to get something, they should. They are taught to hate and abuse anyone who is different or weak. They are united not by love and by appreciation but by fear and action against someone else. This really has to change.
With the exception of those few shameful incidents, I treated other children with kindness, appreciation and loyalty. They tried to walk all over me. They thought they were manipulating me, but I could see right through them all. However, I lacked the courage to actually put a stop to it and let them know. You learnt to keep your mouth shut, anyway. Accusing them of anything was all the excuse they needed to paint you as the villain of the piece as an excuse to attack you. Over the years, as we matured, I was able to influence them, however, and show them my way was better. And in late adolescence, I found many people who were wonderful and friendly. Plenty of people grow out of the childhood nastiness, but most children today are horrible.
To be honest, I'm reluctant to even send my future children to school (if I can afford good tutors it might have to be that*) because, unless things change drastically, I know exactly what they're in for.
* I value scholarship and study very highly, but if they have to suffer as I did to get it, I'm not surte I could let that happen...