Son fighting at school

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Yesterday my son had a pretty bad day at school, refused work & cleaning. he was caught telling some kids he was going to grab a plastic knife & stab them.. today I asked his teacher if he had a better day & she said yeah. He was also much happier today too. I seen he got a lollipop from the teacher as well.. I find out later from my daughter she seen him fighting at school.. physically. I asked him about it & he said it was the same people as yesterday. Saying they were splashing juice in his face.. what would you do in this situation? I wouldn’t want my son to be bullied, & definitely just as much don’t want him to be a bully either.

Just looking for suggestions on this, little back story; we just got custody, he had a very troubled past. He is improving. He goes to counseling for everything, we are currently working on writing incidents down to present to specialists to do some testing on him.