If your child is eligible for enrollment (age, has finished primary education) a secondary school can’t refuse enrollment without a justification. A secondary school can deny enrollment in 4 special circumstances:
In Flanders, if your child was denied enrollment because of one of the above reasons than the school has 4 days to send you a written notification with motivation of the refusal.
For more information:
https://onderwijs.vlaanderen.be/nl/problemen-bij-het-inschrijven#weigert
http://www.agodi.be/commissie-inzake-leerlingenrechten-een-klacht-indienen
In Wallonia, if your child gets rejected, the applicant will be referred to the Enrollment Commission for further assistance. http://www.inscription.cfwb.be/index.php?id=280