Not all that surprisingly, the school has a FAQ posted about the situation where they expelled her. Having read their FAQ, I suppose I can understand where they're coming from -- they say it's because they're partners with the family in educating the child, and a same-sex couple is contrary to what they want from their partner, morally and spiritually speakings -- but I have to say I disagree with the decision. Not just because it appears as though they're treating homosexuality as a worse sin than others, but because I feel they're missing something essential in the character of Christ. He was someone who welcomed sinners freely to his company, and I can't see any evidence in the gospels that he insisted they start cleaning up their act either before they came to him, or afterward. People did that on their own.
Ontario Christian School has chosen to set a purity bar for people to be members of their school community, and they've defined it in such a way that they're saying the parents are members of the community as much as the child, and therefore the decision is made to remove them from the community. Although I'm sure this is a legally defensible position, I don't think it's particularly Christlike, especially since the punishment is being meted out on the student. They can regret that the parents' behavior forced the school into this action, but unless that regret finds outward expression in action, I don't think it counts as more than a semantics exercise, similar to the president's "I take full responsibility."
A private school is free to set criteria for admission, of course; usually these are academic in nature, although I'm aware that some Christian schools use a faith-based screen in order to minimize secular or un-Christian influences. That, I suppose, depends on the guiding philosophy of the individual school, which may or may not be hideously flawed.