Your school will organize a working visa for you. In Ho Chi Minh City it took a few months for them to organize but other countries quite a bit faster. Therefore, if you want to leave Vietnam, for example and go to Cambodia, just a few hours away, you have to get a multiple entry visa so that you can go out of the country and return as often as you like. In Asia, the countries are very close together and you will often go to another country for the week-end. The flights are incredibly cheap on Air Asia and other discount carriers so you have no problem getting around.
Remember...if they ask you what kind of visa you want, pay the extra money and get MULTIPLE ENTRY