What can I do to get my 6 year old child to behave.?

I have a 6 year old child that does not behave in school. I get notes about 3 times a week . In cl he does not show self control ,does not respect others. Just yesterday he told a kid on his bus that he was going to kill him because the kid would not stop kicking the back of his bus seat. I am very irritated because i have tried rewarding him for everyday that he behaves .but it is not working. I do punish my child for misbehaving . He is such a sweet child but seems that he can not seem to behave in cl. I am thinking of taking him a therapist. what should i do?