Eliminating the Catholic boards would not be a budget silver bullet, but there would be some savings.  This may be a small example, but its relevant.  Toronto French public and French Catholic schools share buses.  That is...in the morning, the bus drives through a neighbourhood and picks up the Catholic kids, drops them at school and then goes back to the same neighbourhood and picks up the public school kids and brings them to school.  Sometimes the schools are only a few blocks away and have less than 150 students each.