When speaking Thai is discouraged
Many schools will actively discourage or outright ban teachers from using so much as one word of Thai in the classroom and for good reason. If they wanted someone to teach students about English in Thai, they'd just hire Thai teachers and not pay the premium rate for a native speaker. Because schools want full immersion; students have to listen and try to understand as much as they can, to English as spoken by a Colonial or a Britbonglander, and speak it as much as they can. Same with any language. Hence my Thai ability increased dramatically when I lived in Thoeng, a small town near Chiang Rai. I trained Muay Thai with an Ajarn, who spoke zero English and neither did the local dudes I trained with. Because for a couple of hours a day I was in a position where I had to listen and try to understand as much as possible, and speak it as much as I could.