Hi. I used to teach elementary and there are different things to look at. Like somebody else asked, have there been any changes? Is he a follower and is maybe being influenced in class? I'm assuming he's in K, so maybe it's just an adjustment period. But, I would set up a conference with the teacher and come up with a plan. The two of you might need to communicate daily for awhile. She could send home a small note each day with his name, date, and a smiley or non-smiley. If it's a smiley, that lets you know everything went well. If it's not a smiley, she can write a quick note to let you know what happened. That seemed to help the few students I had who were acting out. I met with one parent who brought his son to the meeting and we let him know together what was going to happen. The days that he came home with a smiley, he got to spend time with his parents playing games or whatever he wanted. The days he didn't get a smiley, he didn't get to play any games. Maybe he had to go to bed early or something. ANyway, you get the idea. We did that everyday for awhile and it made a huge difference. After awhile we went to sending it home once a week and then just dropped it because he didn't need it anymore.
He may also be the type of kid who needs a lot of positive reinforcement. WHen the little boy in my class was acting out, I just simply put him aside and just didn't focus on it. When he was good, I made a huge deal about it. His parents did too.
If his scores are high, he could also be bored. The last thing you want is a bored 5yo in a class! They simply act out. It's hard at that age because it's not as easy to just tell them to get a book and read. But, maybe she can have something for him to do when he's finished with his work.
Good luck!