What does the teacher suggest? She must have experience dealing with other children like your son and should be able to offer good resources to help change the negative behaviors.
In my town, we have parenting classes offered through the school district that are open to parents of all age kids, including preschool. It does NOT mean you are a bad parent at all! It is simply a way to offer you new ideas and suggestions for dealing with negative behaviors. Can you see if your city offers anything similar? I have heard really good reviews from people that attended the ones near me. Please don't think I'm suggesting this because I think you're doing anything wrong - I certainly do not think that. I just mean that classes might help you get the strategies you are looking for.
In the meantime, what does he care about most? A certain toy? TV? An activity that he does? Try taking that #1 thing away and see if it makes a difference.
Also, do you do any kind of reward system with him? Rather than punishing him for negative behavior, try rewarding him for good behavior. Come up with a short list of things you want to change and reward him when he does well in those areas. My son (almost 6) has a chart with 10 rules on it. At the end of each day, we go over the chart and see how he did. If he did well on one, he earns a colored marble. If he did particularly poorly, he loses a marble. If it was neutral, nothing happens. When he earns a certain number of marbles, he gets a reward. Sometimes it is something from the Target $1 section, or it might be a special dessert (going out for ice cream), or sometimes I make him earn more marbles before a reward and then he gets a bigger prize (like a special lego set). Start small - 10 marbles for a reward, then 20, then 50, etc.
A few of my son's rules:
1. Listen to parents or any adult taking care of you
2. No fighting with younger sister
3. Being nice to his sister
4. Speaking nicely - say nice words and say them in a nice voice
5. Don't be bossy
He has a few others, but those are important ones. It works for us. He is mostly a great kid overall, but definitely can act out and he acts better when he knows he can earn the marbles.
Good luck!!