I gave some thoughts in my answer to your related question. I would also like to react to your comment to Rory:
what do I advise my son to do in such a situation. He currently tends to running away crying (at worst with tears) where I'm not sure if it is the best way. For sure, beating back isn't neither...
That is surely not the best way, as it gives a perception of him being sensitive and easy to push out of balance. Which may easily encourage the harassers to repeat their act again, whenever they are bored. The ideal behaviour would be to stay calm, and either
- report it to the teacher (if there is a reasonable hope that the teacher will resolve the issue) or
- just accept it without any reaction for a while (if it doesn't hurt him or make his clothes dirty etc.), to have the offender get bored and leave, or
- give an unexpected or witty response, to throw him off balance.
Staying calm is not at all easy at that age, but you may help by talking through the situation with your son, making him understand that you fully support him, and that there are better or worse solutions to the problem, most of which take time. You may even want to "rehearse" such situations with him playfully, both to help him internalize the proper way(s) to react to such challenges, and to make him experience it in a safe situation, and realize that it is not the end of the world anyway.
Especially the witty approach usually needs rehearsing some canned answer to the given challenge. It doesn't need to be awesomely funny for us adults, it should work on the kids' level. I can't offer a perfect answer to you for this situation right off the top of my head, but share you a few examples from our life: when our daughter told she had been chased by the boys a bit too wildly in the school, we told her to respond to them "come here for a peck on the cheek". That was sure to embarrass those wild boys just trying to show off their manliness :-) In another case, a higher grade student teased her by saying "pacifier, pacifier" (she just started first grade). So we told her to respond "oh, do you need one? Sure, I will buy it for you" :-)
And another thing you may do is to make friends with the offender(s). Invite them for a party, where your kid may present himself in a well known and safe environment, under parental control. Thus, bullying is explicitly forbidden, but fair games - even more "manly" ones like wrestling, throwing balls etc. - are allowed (or even expressly arranged, if the situation calls for this!), but always with clear rules, and a competent parent acting as judge. This may make your son more courageous and earn more peer respect for him, thus potentially ending the bullying. It did work in one case I read about.