Another kid is constantly secretly hurting my kid at school, what should we do about it?

Let me start out by saying I just have my sons side of the story here. I believe him though, he hardly ever lies, and when he tries to lie he sucks at it big time. His face gives him away every time.My son [6 yo] has a classmate [8 yo] who he has been clashing with for quite some time now. The age difference between the two is because the kid was redshirted while my kid skipped a grade. The are both in 2nd grade, but it's a combined 1st-2nd grade class. Which makes my kid pretty average in age and this kid the oldest one there.When they were younger, in kindergarten, they already were constantly getting into fights. They just seem to rub each other wrong. Lately things have been escalating though and the power dynamic seems to be shifting. This week alone there were 4 seperate incidents my kid told me about when he came home.On day one the kid threw mud at my son while outside. Son told him to stop, but kid continued. Then son went to get teacher (this is exactly what we tell him to do btw, first use your words to try to stop it and if that doesn't help go get the teacher). The teacher asked them what happened and the kid told the teacher my kid started throwing mud and then he did as well. My kid said that wasn't what happened. The teacher, thinking the truth is somewhere in the middle, gave them both a talking to.Later that day the kid hit my kid when teacher was out of side for 'getting the teacher'. My kid didn't go the teacher this time.The day after the kid was calling my kid names the whole day whenever the teacher didn't hear and tried to get his classmated to do the same. When my kid went to the teacher the kid denied doing it and said my kid made it up. No consequences for either.Today they had gym class. During the class my kid and this kid got into a fight. I don't know the details of that one. After class they had to get dressed. My son accidentally hit another kid (a 1st grade one) in the face while putting on his sweater. Before he had even the chance to apologize, the kid jumped on him, and squeezed his throat. My kid has marks on his throat to show for it. The teacher came in and saw them fighting and sat all boys down. The little one wasn't particularly upset with my kid and understood is was an accident. My kid apologized. The big kid said he wanted to protect the little one and that's why he started fighting. No consequences again for all boys.Today after kid came home and told me what happened we took him back to school immediately and asked for 5 minutes with the teacher. We told her what we hear at home. She knew about the incident from today, but not all things that happened before. Overall she seems wiling to handle things, however I'm not completely sure things will change. She seems to have a view of 'where two fight two are to blame' and while I generally agree with that sentiment, I think the dynamic has shifted here to where one kid picks on another. Also I asked her directly about the lying and consequences for that kid. She told me it's hard to tell for her what is the truth when she is getting two different stories. While I understand that I think the result is that this kid is constantly getting away with things by lying and that this is reinforcing this behavior (I told her that).So what do I do here? Put my trust in the school?I could also talk to the parents. I know the mum reasonably well, she lives close by. Dad not so much (they are seperated and he lives further away). I don't think the family dynamic there is very healthy. All the kids (kid has a younger brother and sister) are sneaky and get in a lot of fights. The mum is very self absorbed and I think a bit narcistic. She doesn't seem to pay much attention to what her kids are up to. Before you say this would end in shouting matches with said mum, I'm not very concerned about that. I'm a pretty laid back person and usually am able to navigate situations like that well.. I would approach it from an angle of 'have you heard what our kids are up to' and try to gently let her see her sons behaviour is escalating towards bullying.Lastly I should/could work with my kid. My kid is having some social struggles. He's generally well behaved, friendly and layed back. However the social dynamics with groups forming at school etc. seem to fly right over his head. He's a bit of a foolable dork.. It wouldn't pop up in his head that he could ly to the teacher himself for example. Perhaps part of that is him skipping a grade, however he had the same struggles before skipping a grade plus the combined classes at school also make for him not being the youngest currently. He does have friends (in fact, his best friend is a boy where the kid that picks on him also wants to be friends with, so this could be a contributing factor). I don't really know what to teach him apart from what we've been already doing (use your words to show when something bothers you and get the teacher when the situation isn't resolved - he's losing faith there though as he feels the teacher isn't hearing him).Thoughts and advice are appreciated! via /r/Parenting http://ift.tt/2hAzb51