Not sure if it remains true, or if it ever was, but if you leave on the last day of your stamp for a border bounce immigration May count it as a 1day overstay. It makes no sense, but it happens to me several years ago.
If you have any real visa you have to get a reentry visa at immigration or the airport, and I am pretty sure there is no test to get it. Remember, some people get Ed visas to study Muy Thai, which does not involve much if the language.