I know of at least one British guy who secured an admin job in marketing with Central group, who own some of the biggest and most popular malls in Thailand. So, is it possible? Yes. Easy? No. You can also get work with an insurance company or any of the international businesses based here as you can be employed to directly deal with foreign clientele etc.
that doesn't make it legal, many schools register the teacher in an admin role, but if immigration catch you it will be you, not the school, fined and jailed.