Let's just get this straight from the start. All students are capable of learning. Except for the few with cognitive disabilities, all of them are capable of working at grade level, which is regarded as developmentally appropriate. In fact, they are capable of far more.
Some students have been well-prepared for school and some have not. Those who have been well-prepared need to be challenged to meet all of the minimum grade level expectations and then go beyond them. Those who have not been well-prepared need some additional support to allow them to also meet all of the minimum grade level expectations and then go beyond them.
Every student is well-prepared in some ways and to some degree and poorly prepared in other ways to some degree. In short, they are each unique individuals.
Identifying individual student needs and providing those students with the lessons and support they need is the professional work of teachers. Teachers employ their training, their expertise, their talent, their effort, and their