Best place to study English in Medellin?

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Hi,

I have a couple of friends who are going to be living in Medellin for 6 months next year, and they'd like to use that time to study English, at least to start a good course, and they asked me to recommend them a school, but frankly I have no idea. What's your experience and what schools do you recommend (or advise me to avoid!)

See you Laneros!