This is perhaps the most difficult part for me as an instructor, as I have expectations that students want to and need to be in the class or in school. I've worked for schools where the classroom door was expected to be locked after the start of the class, and everyone told not to answer the door if a student knocks to get in after class starts or if tardy from coming back from break. The rationale is that students coming in late are disruptive, when I find it just as bad to stop class and tell a student to sit down and not answer the door. I find it defeating. I'm not actually sure if locking a classroom door is even legal!