Hi bxl-forever and RaphaelPasloin_mobile,
Thank you for your comments and sharing insightful references. They have deepened my understanding on bicycle traffic regulations in school zones in Belgium.
As I understand, keeping motor_vehicle:conditional key would imply bicycle traffic is allowed, but along with other non-motor vehicle traffic as well. 
Alternatively, vehicle:conditional key in combination with bicycle=yes would imply the exception of bicycle traffic, while prohibiting other non-motor vehicles still. 
Which, do you suggest, would be the best way to tag the street, based on your local knowledge and mapping experience?