Throughout middle school and the first two years of high school. I think my worst memory was when the bullying started. Up until third grade there was really no bullying. There was this one kid who was best friends with me in third grade--we were inseparable all year. We went to each others' houses almost every day, and we'd always say how we were best friends forever.
Then fourth grade started, and immediately things were different. We started separating into a social hierarchy and because of my big nerdy glasses and general awkwardness, I was with three or four kids at the bottom. One day when a bunch of kids were picking on me, and I went to my "best friend" hoping he would stick up for me, he literally was like "You're not my friend!" and joined them in picking on me, and one of those kids became his new best friend.
I know it was self preservation--he didn't want me to drag him down, so he attacked me to keep himself out of the "nerd" group. I understand it. But still... says a lot about loyalty. I feel sure I would never have done the same if the situation were reversed.