Peer pressure is not a constitutional issue. The peers are other student. Private individuals. It only becomes a problem when the government (teachers in this case) do the pressuring.
Peer pressure is a constitutional issue in the following context: a school decides it wants to impose daily prayer on all students. Each morning, the students are told that there will be a one-minute prayer. They do not have to join in. They can stand silently if they wish, while "the rest of the students" recite the prayer.
The school knows full well what effect this will have on the non-praying students. They will feel peer pressure to join in. Hence, the school is seeking to accomplish in a roundabout way, what they cannot attempt to do directly. In other words, the school is using the peer group pressure they KNOW will exist, in order to force non-praying students to start praying.
A school cannot USE praying students who they know will exert pressure on non-praying students, as a method of forcing prayer on students not otherwise willing to accept it. In other words, teachers cannot indirectly do the pressuring through the medium of one group of students exerting pressure on another group of students.