A language school will send you to teach company staff at their workplace. You might be teaching report writing to a group of five managers or basic business English to ten giggly secretaries and receptionists. Most companies will study a couple of nights a week, say Tuesday and Thursday - and sessions usually last about two hours.
I wrote this article on corporate training a good few years ago but little or nothing has changed since then. The article will give you a good overview of what corporate training in Thailand is all about.