It is usually the case that the bullies are troubled kids, in which case the kid bothering your son needs some help, too. I don't think it would be so hard for the school to do its job in this instance. A little extra supervision of the bully should work.
there is another complication - we chose to put our children in a very integrated school. and we live in a district that is very integrated. the school in usual - about 50% upper middle class, about 50% barely making it into middle class. so, in pretty much every class you have half the class with well educated, mostly 2 parent families, often with the mother staying at home. the other half are really struggling to keep their family in the middle class, often it is a huge struggle. many single parents, many parents working long hours.
the other kid falls in that group. I don't think he is a bad kid, he hasn't had the same advantages my kid has. but that doens't mean I want him to distract and bug my kid. I wouldn't mind their being friends, if that worked out. but in the meantime I don't know what the teacher will do - say to his parents "hey, maybe you shouldn't work so many hours and spend more time with your kids and by the way, maybe you should read up a little on child rearing and be better role models for your kids, so that globe doens't have to suggest to his kid to kick your kid in the face"