Young people can communicate and develop together with their peers, but they can not give each other new knowledge. Teachers are much more experienced and educated, therefore they can give a lot of necessary information. Only they can not so clearly understand the language to bring it to the children.
Very few are given the gift to communicate knowledge to children. This is really hard, because children do not want to study, and a competent teacher who knows how to do this, always finds an approach to every child. Pedagogy is art.