International schools are very expensive, we could only access them due to job package. Europe School system was much cheaper as set up for the children of EU employees kids - there are 3 in Brussels I think. Our DC went to the European School in Munich when we lived there. Most pupils there long term were children of parents working for the European Patent Office.
Not sure about entrance criteria since Brexit but these schools were previously very good at catering for free movement in the eu, especially for English, French, and German mother tongue. Additionally, they legally have to provide some tutoring in the mother tongue to any child from any member state