many years ago when I was first teaching, I had a "problem" student. I sat down and talked with my teacher and gave him all of the specifics. Instead of providing me with a specific answer, he told me that I was responsible for the safety and well-being of the class. This is also the responsibility of your teacher. If your teacher has not yet stepped to the forefront to handle this situation, that all of the seniors need to set down AGAIN with the teacher and ask that the teacher DIRECTLY deal with this situation, so as to prevent students from having to act in ways that may not be consistent and may not always be appropriate. The goal is the overall student body safety and well-being. This should be the over-arching basis for direct action by the teacher. If I were a senior student, I would make that clear to my teacher, particularly when the safety and well-being of the students can not be assured when this person is attending class.
Good idea Sensei! I will be keeping an eye on this student. I will get the senior students together should something happen again.