Have to disagree with you Suzy, parents cannot make their children behave in school. The school must do that. I am a teacher and if a child is misbehaving in class I will discipline him. I don't expect the parents to have to punish him/her again. I wouldn't like to be punished twice for the same thing. I give out a punishment i think is fit for what has happened. Likewise, I wouldn't expect to be discipling a child for his behaviour at home.
Parents can make it worse but not supporting school etc. I hope tha parents back the school up but not don't punish him/her twice.
I recommend making an appointment to speak to the teacher.... ask what the strategies are in place, why they aren't working, what they intend to do now. Also ask if they think there is underlying cause. Talk to them abut his low self-esteem, maybe they have some strategies to help this.
It is true that teachers have 20-30 kids to look after, so maybe you need to talk to the school about the standard of behaviour they expect in class, so maybe you can discuss it at home with your son and explain why he has to behave in a certain way.
Good luck worried mum.... just be assertive with the teachers. If you and they work together, you can turn the situation around for your boy.