Hi Sandra,
Your son is still young. You are obviously concerned by his poor behaviour, but if this behaviour is only at school, it is the teachers who need to get him to behave , not you. You are not there. You can encourage, and support the school, but at 5, he will not remember what you have told him when he is about to be silly at school.
I suggest you make an appointment with the teacher, and ask him/ her what starategies the school is using to help your son learn to behave. They could have a jar that gets a marble put in when he behaves and taken out if he is naughty. When it is full, he gets a reward. Golden time works with many children. The school should have strategies. It is their responsibility to teach him to learn. It is your responsibility to support the school, to ensure he has enough sleep, food etc and is cared for so he can learn.
Please make an appointment with the school to discuss this matter.

Oh please read my other post on charts and rewards. These are not the best at all and lead to aggression. He is only five. Five year olds don't behave bad. They do things innocently and can't control themselves. The teacher must figure out what is not working for him.
He may need a different set up or activities, or just to stay at home with you.