A teacher not being there 30 minutes before class, absolutely acceptable. A teacher being 30 minutes late repeatedly (*cough*oneofmyteachersnotsayingnames*cough*) is bad.
Anyway, the worst part about my school is they lock the door 30 minutes before schools starts, not 30 minutes until school starts, 30 minutes before. It's so annoying because they won't give us the door code, and we have to buzz in to get inside.