The feature of any era differs. In the past, students regarded teachers as a symbol of truth, and they would always follow their teachers with implicit obedience. That really made the teachers have a enormous influence on their students. But for today, as the development of online technology promotes the communication between teenagers, students are more willing to take part in a variety of interpersonal activities and learn things in many ways, which makes them easier to be influenced by their contemporary around. So I believe that students are more influenced by their friends nowadays.
At the outset, we all know that the limited ways of acquiring knowledge in the past made the teachers leave their students a charming impression with infinite knowledge and elegance. Teachers, which means knowledge, gospel and truth, enjoyed a significant high level of statue in the society. For example, students must kneel before their teacher at the first meet as a ceremony in some Asian countries like China. In some profession, students even had to take care of their teacher in the daily life. With the excessive admiration, students would strongly tend to imitate their teachers' behavior and their way of thinking, which gradually fostered their characteristic. As for friends, there were scarcely social activities in orthodox school in the past and students lacked the chance to communicate and keep in touch with their contemporary. So teacher are definitely the one who influenced their students the most.
As for today, the situation gradually changes. The unprecedented advanced communicate technology really links our contemporary teenagers together. Students can chat by words and videos online, and the increasing life quality in urban family makes it a lot easier for friends to organize outdoor activities together. Students can gather together to have a picnic, play soccer or enjoy a home party in their holidays, which will tie them together and unconsciously promote their behaviorally and psychologically consistency in the way of thinking and dealing with problems. I have to concede that teachers is still the most important and irreplaceable part in classes, but it is all too easy for us to find that students are more likely to imitate their friends around and act the same when treating things instead of following the instructions proposing by their teachers. And most of teachers will readily give enough private space to their students, which naturally form a gulf between two generations. And both students and teacher are willing to find their right situation in that gulf. All these changing lifestyles and attitude can show that students are more influenced by their friends but not their teachers today.
In the nutshell, it is an open question whether students are more influenced by their teachers than their friends, a question should only be settled in specific era. I believe that teachers were the most essential factor in the past, whereas friends plays a more important role nowadays.