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

My advice is to contact the school immediately.

I don't know about UK, but here the school has to take action if they are notified about such things. And it's my experience that it's also very efficient if action is taken early and correctly. If the teasers are young, the teachers usually organize a role play for the classes involved, where the pupils all get to feel how it feels fo be teased for a few minutes. The teasers are not told about this, the point is that they shall get to feel this without getting to know that this is really about them. The teachers will also tell the classes that the teasers usually are the losers, that they are cowards who will never tease when they are alone, that they usually are jealous and so on. It's really amazing how efficient this is, I have seen it myself in many cases.

My eldest son had one of these teaser in his class, and my son was the target. Since I knew the parents, I contacted them, but they were not able to get him to stop, instead it only got worse. Then I contacted the school, and they tried this role play a few times, to the children they called it a game. To be honest, I was shocked how quick the change was. A few days later this teaser became one of my son's best friends. As is usually the case, it turned out that this boy felt left out if the group, and started this teasing to get accepted by the other children. And "boys are boys", when one started, the others immediately followed his example.

One last remark. Contacting the teasers directly or asking the school to talk to them, usually will only make matters much worse.