My 4 year old is having problems at school

My son is 4 and a half and is amazing at home. Occasionaly he may have to go to time out at home for not listening or talking back but that is about it. He is a normal boy who loves to interact with anybody and everybody. He is rough at times with some girls but usually understands that he can't be rough with them. The problem is at school. He is in pre-k and is usually pretty well behaved. Nothing out of the ordinary of a 4 year old. Here recently the last two weeks he has been in trouble every day. He is not listening to the teacher, talking back, kicks or hits other kids, runs away from the teacher in class, and is generally been disrespectful. Last week when this happened I would just put him in time out when we would get home and then just talked to him everynight about what good behavior is and why he is choosing to make bad choices. I kept telling him how good he has been and how good he is at home and that we need to take that good behavior and attitude to school and not just home. I have asked him repeatedly why he is doing this behavior at school and i get the normal 4 year old answer of either "I don't know" or "because I was mad". He will tell me what the right way to react when he is mad all the time but will not do this at school. This week his behavior has continued. Last night I would not let him watch any t.v. and we sat in his room and talked for about half an hour (which is an amazing feat for a 4 year old) about behavior and choices and really thought it was finally clicking. I called his school today to check on his behavior and it did not improve at all. He continued being disrespectful. I am at my wits end. I have no idea what to do. We do a sticker chart where if he gets stickers for school and for home when he is good and at the end of the week we get to do something of his choice. That worked perfectly all of October and for this month its like he doesn't care, Although he still gets upset when he doesn't get a sticker. Please help!