I don't work in a school, I work in an office, for a medical doctor originally from the UK, to provide some context. When I started with the company about 11 years ago, you would never see the boss without a jacket and tie. The staff would dress accordingly on the days he was in the office, which was only once a week. By "accordingly", for me that meant khakis and a button-down shirt. Times change, and I haven't seen my boss wear a jacket or tie now in several years. I still wear a button-down shirt on meeting days, but it's more of a casual shirt that I leave untucked, and I'm permanently in (nice, clean) jeans, and on hot days when I know the boss will be away, I'm not above wearing shorts and a t-shirt. And sandals. I've seen teachers at my kids' schools wearing shorts. Casual Friday creep, I guess.