Education just does not work the same way for all young people. Some are not as naturally gifted as others are, for instance. In addition, some come to school with family and economic problems that make teaching very hard. That is why schools need to be honest about what is not working to find out what might work.
In order for any school to improve on its performance, the school and its administrators must be honest with themselves regarding the school's failures. This includes whether the school has been able to encourage so-called slow learners to the best of those children's abilities. Ignoring inefficiencies hurts children schools are supposed to help.