I am a teacher in a school with blazers. On the whole, students come into the room, put down their books, remove their blazer and sit down. This is fine with me. On rare occasions, students come in a bit rudely, maybe continuing a corridor conversation or generally being a bit out of learning mode, and I will then say sarcastically, "Yes, you MAY take off your blazer", with a bit of an eye-roll (yes, I can be as childish as them haha). Then I remind them to put them on before they leave and, apart from a few girls who would rather do anything than wear their blazer, they do so without any fuss and troop off.
Wearing them in the corridors is another matter, though. Boys never seem to be a problem, but girls...