Some very good advice from specialmom. A word on discipline. At this age, discipline has to be immediate. There is almost nothing you can do at home that will carry over to the next day. You do need to let him know that you don't approve. But to place him on bread and water or long timeouts, etc. won't work. The school needs to deal with the here and now. If this is happening at home, then you need to deal with it. Somewhere about 3rd grade, then longer term cause and effect will began to sink in.
So you need to deal with this in other methods than discipline - which is why the above post is worth reading.
Hi, our kids can sure stress us out, can't they? Anyway, I'm sorry your boy is having a tough time. I'm saying that because he probably is inside. He may be very anxious about this transition to school and all that is now expected of him. He may have a nervous system problem that makes him feel uncomfortable as well and he acts out. (noise bugs him, crowds, overwhelmed in general so his nervous system is flooded and making him act out.) He is still only 5 and if this is a whole new enviroment for him, that is a lot to get used to. Is it all day or half day kindergarten (i believe in half for the reason that a whole day is often too much for kids . . . but others have different opinions on it.) I would suggest being proactive and calling his teacher right away to strategize for your son. What types of discipline is she using, what preciptates the event, etc. Maybe some simple changes of classroom dynamics would help. Often seating a child that is struggling right by the teacher helps or giving him special jobs during the day helps. And if there is a known stressor, working on helping him with this would help. Also, keep his physical activity high. Make sure he is running, jumping, climbing and swinging even outside of just recess. Then if need be, get an evaluation done at school. Public schools in the US must provide this and then make sure they provide services to a child that needs them, no matter how minor the need. This is a law. Your son can not abuse the teacher or other kids, and they and you must stop that. However, I'd look at what is underlying it all. He may feel as bad on the inside as his actions can be on the outside. good luck.