For pre-primary, primary and secondary education, the school year starts on 1 September, or the first working day after it and ends on 30 June or the last working day before it, when the summer holidays begin. There are slight variations among the different language communities (principally for public holidays observed by an individual community) but essentially school holidays are the same throughout Belgium. There are three national holidays in May and often an extra day is added on to make a long weekend break on at least one of these.
Some international and private schools have different holidays or may observe additional national holidays.
https://www.angloinfo.com/how-to/belgium/family/schooling-education/school-year