As you indicated, today's public school classrooms tend to have a wide range of ability levels within each classroom. Learning to deal with such a variety of strengths and weaknesses is challenging for the teacher, as well as for the students themselves. It can be a juggling act, but each teacher learns to adopt curriculum and teaching methods that assist her in facilitating learning for all students regardless of their abilities. Your child's teacher has suggested a method known as peer tutoring to help balance the classroom. The students who have strong writing skills will be helping others in the class to improve their own self-editing skills, which will have benefits for everyone involved.
Kids learn well from other kids.
Sometimes children can learn a concept from a peer more easily than they can from an adult. If so, that will give many other students in the classroom the chance to act as tutors, but still may not allow leadership opportunities for all. I can see where this would be of concern to you because the teacher indicated a strong belief in developing self-esteem and leadership qualities in each of the students. Clearly there is a good chance that not all of the students will be able to act as peer editors or tutors. However, the teacher may have additional plans for those students who might not otherwise have the opportunity to act as leaders.