1. Possible dismissal: courses with strong professionalism, such as medical courses, chemistry courses and biology courses that require the use of instruments and equipment. Students who are not in this class are generally not allowed to take such courses, because even if they do, they may not be able to understand the course content because of the lack of necessary equipment. Small class specialized courses. The number of students in such courses is small, and teachers can often remember each student's name. Therefore, if an observer suddenly appears, it may attract the teacher's attention and confusion. Extra paid courses. This kind of courses usually require students to pay extra, such as training courses for maritime business majors and training courses for computer secondary certificates. If non-professional students try to participate without paying, they will generally be rejected.
2. Generally, it will not be driven away: public classes or large classes. There are a large number of students in this kind of course, and teachers often can't remember each student's name, so even if there are students in attendance, it is unlikely to attract the teacher's attention. Courses that teachers are allowed to attend. Some teachers may be more open and tolerant, and let students from other majors or interested people come to their classes.