They allow undocumented students? Who has to pay? I think you are confusing Canada with the US. The US has turned a blind eye to illegal immigration to the point that they decided that they will not ask any questions about kids enrolled in schools. I have also not heard that anybody has been asked to pay to enroll their kids in school in Canada.
The two cases I know of: One I saw on TV, the family had come over on a visit visa hoping to arrange jobs and work permits, not the smartest thing I know. They tried to enroll their son in school and were told that they had no right to enroll him since they were on visit status. They were in Manitoba I think. They were not offered to pay to enroll him, just told that they can't. The other case was a woman on this forum who was taking care of a foreign boy. She had enrolled him in school, no questions asked but maybe the school did it because she had status in Canada and she may have had some papers showing she was acting as his guardian.
So.. I think it depends on the school. If anybody is considering enrolling their kids in school while they are on a visit status. It might be a good idea to contact some schools and ask.