Children need the right to learn in their mother tongue. The outcomes are dramatically better than if they are deprived of it and forced to learn another language while also being held responsible for learning material presented only in a language they do not yet speak. In practical terms, they can't do both at the same time. They learn the language -- if they're young enough -- and lose most or all of the content that teachers supposedly presented. By the time they learn enough of the oppressing language to decode actual lessons, they're six months or a year behind, if not more. So then they can't catch up. They just muddle along until they leave school.