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:
The school is full. Enrolling is on a first come first serve basis, if the school is full you will be put on a waiting list.
Your child was permanently expelled from this school (expulsion can last up to 3 years)
Your child was expelled from a different school in that same academic year. In this scenario your new school can only deny your child enrollment if the previous school and new school have made an agreement on this at the LOP.
The school possesses clear evidence your child variably attends different schools.
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