This is a big pet peeve of mine.  Students who just walk out of class assuming that I am fine with it or don’t see them.  Yes, I do see you!  It’s rude and it concerns me because I am responsible for your welfare!  I think much of it is their belief that they may just come and go around a teacher who is not the “real teacher”.  Perhaps they want to see if I will let them go or if I will actually say something to stop it.  Recently, there was a student who enjoyed particular freedoms and decided to leave the classroom and do other projects knowing that I was the teacher.  It disappointed me because this student is a responsible and smart student.  I felt disrespected but my SA had a talk with this person and made it clear that it is my classroom now.  I know now that I can be more stern with these students because I always stick to my word and maintain high standards.