>So, what do you tell your 11 year old, or otherwise do, when / if they get teased at school by several other kids?

Advice him/her my daugter's way. She said me at once:

"Dad, if someone teases me, I override it. That makes it angry and after saw failed tease won't try it again.".

In a few other cases my daugter makes a counter-attack. My daugter is a dangerious girl... :)