The instruction comes in a three page government circular outlining security measures they should take in the aftermath of yesterday's bombings in Brussels.
Schools were open as usual this morning, but education minister Joëlle Milquet has written to headteachers saying that schools should have just one point of entry to their sites, which is guarded at all times, the RTBF.be website reports.
At the end of the school day, staff must oversee the handover of children to their parents, and be able to call the emergency services in the event of an incident.
Large gatherings in front of schools should also be limited, the circular said.
Heads have also been advised to make contact with the local police force in their area to arrange any further appropriate security measures.
Brussels schools have been told to cancel school trips for the time being, local newspapers reported.
Schools received the circular as terrorist bomb attacks at the Zaventem airport and a metro station killed around 34 people and injured many others.
The local government leader for Brussels, minister-president Rudi Vervoort said that despite the attacks and a threat level of 4, there was currently no lock-down in Brussels.
“The objective is to return to normal life this Wednesday morning”, he was reported saying in Belgian newspaper Le Soir.