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LOOKING FOR ADULT BALLET CLASSES/TUTOR
Try Danz Royal for English.
This place is likely to have English too.
http://www.brusselsintballet.org/BIB_SITE.html
The local French music academies have 11 levels of ballet, the highest is Q7, it's the same levels throughout French Belgium and Dutch speaking Belgium has a similar system of music academies with ballet on the curriculum. Try a few of them to find out how high the highest level is aimed at.