Teacher is the orcheastra conductor who organizes everything in a class. He has the responsibility of what to teach and how to teach in general. It is the same thig for vocabulary teaching.
In contrast to private courses founded for commercial purposes, at shools students have very similar aims for learning foreign languages. Besides they are the teachers who lectures the advantages of knowing a foreign language to students and where students use that language. After some time, students motivate themselves by focusing on the reasons to learn a foreign language. So teacher mainly knows where their students will use that foreign language, let's say English, and can have a standard in choosing vocabulary to address all students' needs.
From another point, English teacher is a person who learnt English for some reasons and he is the master in this field. I think he is the person who can achieve different purposes with his knowledge in English. So he is the right person to decide the order and the group of vocabulary to learn. He came to a level which his or her students will come later on.
Anyway, teachers use a prescribed curriculum or sllybus and also coursebook. It means that the vocabulary to be taught students are already chosen. So a teacher just has a chance to persent them in a different or more meaningful context or reorder them and teach them at different stages according to the level of students.