To be fair, the school shouldn't have barred the kid. The parents were the problem.
“The standoff reached a climax in June 2015, when the school asked Mangel and Yasué to sign off on their acceptance of the curriculum, which emphasizes multiculturalism. When they refused, the little girl wasn't allowed to return to school in the fall, according to the decision.”
Seems like the parents had to sign off on the curriculum for the child to attend, and as they wouldn’t, she couldn’t.
Their demands are that the child should not be exposed to any religious or political events, so a more extreme version of Jehova’s witness, but unlike JW’s, they want the school to stop, not an exemption for their child.
There was absolutely no situation where this comes out good for the school, they are absolutely stuck unless they allow this specific set of parents to prevent/use their child as a proxy to campaign against everything from recognizing a variety of cultural (ie different cultures) events to national ones like Canada Day and Remembrance Day.
The court gave them no suggestion as a solution, no mediation for a way forward. They are right back where they were, because our HRC is a joke.