I agree with PP. Get in to see your pediatrician as soon as possible for an assessment. This sounds just like my DS when he was 5. Hitting, punching, kicking. No form of discipline worked at all. No self regulation of frustration. Kindergarten was a nightmare. We got calls from the school multiple times a week. He was on the verge of being suspended. He was diagnosed with ADHD by the pediatrician right before he turned 6. We put him on meds which helped tremendously. A year later, we got him in with a psychiatrist/psychologist team to better regulate his meds and start therapy for him and us. The therapy was a tremendous help to us as it gave us advice on prevention of his outbursts as well as what we could do to minimize them when they happened. DS is 9 now and is doing well. We still have our challenging days but he doesn’t get in trouble at school anymore and is more self-aware of his ADHD and is able to advocate for himself at school.

Good luck to you and hang in there!