This is a very serious situation and the school along with you should complete a Functional Behavior Assessment to determine the triggers of the behaviors and then complete a Behavior Plan to address the behaviors. You need to advocate for your child, he is acting out for a reason but this acting out is/will impact his success among other things.
I would call an immediate meeting with the teacher, principal, school social worker, psychologist and get moving on what I mentioned above. No 5 yr old child should be suspended 4 times two months into the school year.
Lots of luck and please be persistent.
why are you fed up with the school? do you believe your son over the school?
Some kids do have impulsive reactions to being hit. Some kids, in response to being hit, will turn around and hit back without thinking. It is an instant, emotional response. It is not right, but it does happen. These children need to be taught to control these impulses and be given the problem solving skills necessary to deal with these situations.
If your son is telling you the truth, then his situation is different. If he is telling the truth, then his responses are not the typical impulsive reaction of a 5 year old, but rather a calculated and vengeful act. In my opinion, this is of much more concern than the typical aggressive situation. He desperately needs to learn appropriate responses and problem solving skills. What have you done to help him with this? The school should be working on this as well, but saying you are fed up and blaming the school and the other children is not the solution. I suggest you schedule a meeting with the teacher and/or principal immediately to discuss the situation. Ask them to record the incidents- where they occur, time of day and what happens. This can help you look for patterns and identify where the problem truly lies.
in the meantime, discuss these incidents specifically with your son. Ask him what happened. Have him identify how he felt. If he claims someone hit him, ask him if he knows why. Discuss what he could do differently next time. You can validate his feelings, but do not ever let him think that it's OK to hit back. "You were angry that Mikey hit you. What could you do next time?" You can even role play scenarios with him.