That's just little boy behavior! They play rough. Sometimes daycare workers don't want little boys to be little boys because they fear injuries on their watch. Little boys play fight, wrestle and roll around on the ground and do kung fu kicks with each other. It is not bad, it is just different from little girls.
There was a study that showed that even when mom's and childcare workers tried to keep toy weapons away from boys they still found ways to pretend they had them--even to the point of eating a piece of toast into the shape of a gun. However, they also found that in the kids' play they were not pretending to be a bad guy, they were usually the good guy, saving their friends, family or the world.
Kids used to play cowboys & Indians, Cops & robbers, Superman, Spiderman & Batman--and there was no problem. It didn't make them grow up to be killers. Now it's just not "PC" to allow little boys to play like little boys. My brother was an only boy and he would fight invisible foes!
I would suggest that this happens at school because being an only child, school is where he finds other little boys who understand "boy play." If you don't want him to get in trouble at school just ask him to cool it at school and invite some of his school friends over.
If they are not getting into real fights, I don't see any problem with their behavior. The phrase "Boys will be boys" actually dies apply here.