According to my interpretation of section 23 of the Canadian Charter of Rights and Freedoms, there just has to be one. If a francophone child needs education services in French, that child should, in principle, have access to them. That said, from an economic standpoint, it's not practical to open a school and hire a teacher for one student. However, we need to recognize the value of these children, even though they are part of a small community. There are small communities that receive very good education services. I always hesitate to give a specific minimum number, because all situations are different and all students deserve to be educated in their mother tongue.
