Consider effects on student self-esteem as well as achievement. If students learn in school that no one of their gender or race ever did anything worth studying, they may eventually decide they don't want to learn any more history or science or even come to your school any more. If they have to sacrifice their self-esteem for achievement in your schools, maybe it's just not worth it to them. If a student continually fails, he or she may conclude that it is better not to try. I would recommend that every classroom should have a mix of experiences so that every child experiences SOME successes.
Every child is good at something.
There will be failures, too, and that is okay. Overcoming your failures, and achieving something you weren't always able to do gives a sense of achievement and also boost self-esteem. People need a mix of success and failure experiences.