The following aspects should be on your checklist:

the number of course participants (maximum and average)
the duration of a lesson (usually 45, 50 or 60 minutes)
the quality of the leisure programme (included in the cost?)
social activities accompanied by the teachers (included in the programme?)
school transfers (included in the cost?)t
teacher qualifications in specialised English (relevant for courses in work-related English)
number of meals provided
type of accommodation (some schools can offer "executive families" for adults who value a high standard).
juniors are often given a choice between single and twin rooms
schools with boarding facilities tend to be slightly more expensive
costs for registration and learning materials
discounts or special offers, especially for long-term students
seasonal fluctuations regarding price and number of course participants

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