We've had similar problems, and some not-so-nice teachers and kids who made it a lot worse. In DD's case, in 3rd grade there was one girl who was really bullying most of the other girls and the teacher was oblivious to what was happening and IMO was even used by this one girl to wield power over the other girls (by getting them in trouble with the teacher, even though she had initiated the whole thing). At first I thought it was just DD making stuff up but later came to talk to other Moms and found several who explained the situation to me, which fit with what we were seeing too. Unfortunately this teacher was absolutely unhelpful in doing even the simplest things to address the problem--for example, we politely asked that DD maybe sit at a different table for lunch since she was having so much trouble getting along with this girl, but no, we were told, we should instead help DD 'bullyproof' herself and they were not going to change the lunch seating in any way. And this was a private school, and not a cheap one. I am still just baffled by how unhelpful they were in this situation, especially since we had had great relationships and experiences with two sets of earlier teachers at the same school.
Anyway, sorry to go on so long--IMO, I would consider politely approaching the teacher and mentioning that your son seems to be having difficulty getting along with this one kid in particular, and see what she says. Maybe she would be willing to sometimes intervene to keep them apart, and maybe that would help your DS not get upset about other things so much??
Good luck!
