I have noticed over the past five to ten years a shift from the “semi-formal” attire to mostly the business casual look for male professionals here in Thailand, with the full suit and tie look reserved for top executives and the occasional formal occasion. It seems like we have been following the US West Coast trends.
I prefer this trend myself, as I think it is much easier to look smart wearing business casual clothing (slacks, professional shined shoes and either a button-down short- sleeve or Polo-style shirt) than the old-fashioned semi-formal look of a long-sleeved shirt with a tie. It is also more comfortable. Take a walk down the business district of Bangkok during working hours and see how many men are wearing long sleeved shirts and ties. Not many, although it was the look of 10 to 20 years ago.
I personally prefer the business casual look, it looks more confident, but still professional enough to set one apart from the common tourist.
Personal opinion, while a man can look very professional and smart wearing a full suit and tie it needs to be worn in the right situation, the semi-formal old fashioned long sleeve often white shirt with a tie without a jacket looks pretty dorky on most men, although some can pull it off with style.
I am not sure what is necessarily expected at government schools, but I suspect most language schools and other places operating outside official government regulations are comfortable with male teachers wearing clothes of a business casual style, which in my view is both more comfortable and looks smarter.
But of course, this is just opinion, there does seem to be some national differences with some of the English (British) teachers generally preferring the old-fashioned semi-formal look while many American teachers preferring a less formal look, with Canadian, Aussie, Kiwi and other foreign teachers being split on these opinions.
Just an observation.