Finding a proper language school is kind of a task - even if you roughly know in which part of the world you would like to hit the books again. The amount of offers is overwhelming and there is hardly a useful review in the internet.
Here are the 6 things you should think about before booking.
1. Choose country and region
This one seems obvious as you might already have a destination in mind. But most languages are spoken in more than one part of the world and although all are native speakers, the spoken language differs greatly. Think of UK and the US. Or Spain and South America. Even different regions of a country mean different dialects. You probabely would agree that a guy from Texas speaks VERY differently to a New Yorker. So when looking for a location, keep in mind that you will learn that dialect and adopt the accent. You might not want to complicate your learning and later communicating by choosing a region with a hardly understandable dialect...
2. Comments and reviews in the internet
The first thing I do when looking for, lets say, a hotel, is reading some reviews and trying to get an impression. Now everyone knows that those comments are to be handled with care as they might be an exception or even fake. Comments about language courses are not different, but on top very personal. Well, what else should they be, but "my family was great" or "the teacher was crap" was not really helpful for me. This - or the opposite - could happen almost everywhere and depends also a lot on you and your personality. I recommend to check the internet pages of the schools and contact the ones, that are shortlisted, to find out which one suits you.
3. Location of the school
Find out, if the school is located in a possibly dangerous neighborhood. This might not be too serious as you probabely would be in school in the morning - even junkies and criminals need to sleep eventually... ;-) But you might need to take the bus/train to your course early, when its still dark. Just saying.
Schools often help to find a place to stay. Some have student homes, some find a host family for you. Some even convey luxury apartments with cleaning service. Of course you always have the option to find something on your own, it is all a question of money. Accomodations offered by the school are often close to the school, what might not necessarily be an advantage. I read about a young guy, who stayed with a family in Candelaria in Bogotá, which according to a friend is not the best place to be after dark. They told him to be at home at 8pm, which made him feel like he was underage, but he didn't get that they just didn't want him to get shot.
We booked our apartment via airbnb.com, as this was simply much cheaper than the one offered by the school.
5. Additional offers of the school
Most schools offer a variety of additional classes (like dancing and painting) or cultural activities (like excursions, concerts, sport events, even charity work) with desperate but hopeless intention to make students practice the language. Some activities are included, some need to be paid extra. As you will have some free time, this might help deciding in favor of a school.
6. Book the school directly or book via a broker
In most schools you can sign up for a course by filling out a form on the schools internet page. On the other hand several agencies specialized on language travel offer the same courses at the same schools - for the same price. Some agencies even offer a discount to the schools price. So which way is better?
If you book over the schools homepage, the invoice is send to you via email and you need to pay with credit card or wired transfer to the schools bank account (foreign currency). You will communicate with the school via email and probabely in english, as they might not speak your language. You will have a contact person and a confirmation from the school itself.
Using an agency gives you the chance to actually talk to someone in person. You will pay the agency in your local currency and will probabely have more payment options. You might get a discount (e.g. here) and you have a third party envolved who you can contact, if things go wrong. Provided that the agency is trustworthy. We chose this option after contacting the school itself via email. We paid the same price but the lady at ESL talked a lot about her experience with the schools and feedback from former students. That tipped the scale for us.