Before age 6 they are (almost) all at it so often that the teachers would go mental if they had to inform you of each skirmish. By age 6-7 most the time the children can tell you what happened, the teachers don't need to. If the blow left a mark/bruise/swollen bump I would expect teacher or TA to say something about measures to prevent in future.
I fear that DS is one of those children who often randomly thumps others and I am never told about it, (NB: no one has told me DS is doing that); I just know from what he tells me that he at least occasionally thumps and gets thumped, and if the school generally doesn't mention thumping it could be happening a lot more than I know. Not that I can do much about it in either case.