There is no law like this. You must have heard wrong or you are making up a story. When I saw your post I was happy you provided a positive experience but I doubt very much that two officiers from the PSB told you that you can not work directly for a school. I do know that some public schools in small towns are able to hire teachers through agents. It was explained to me once that some rules are overlooked to help the poorer countryside schools. Most Universities and large language mills do hire directly.