Isn't it too late to change schools at the beginning of the school year?

No, it's never too late to change schools. And there are no universal criteria for knowing when to do it, either. There are obvious markers: teachers yelling or labeling a child, school bullying, and the administration not responding,u can learn more here https://smapse.com/catalog/switzerland/boarding-school/ . There are less critical, but equally important signals. "If a child has no friends at school for a long time, that's a reason to think about transition. A child can survive almost anything, but not the lack of friends," . She also advises asking about favorite subjects: if they are there, it means there are favorite teachers at school - this is also important.