I am trying to figure out what has happened.
I don't think you will be able to sue the school board successfully but here are my suggestions.
If your child was accepted and they knew the whole story in terms of her IPP and what her needs were and then they turned around and changed their minds you have a strong case for pushing to get this rectified.
If they didn't understand what she needed in terms of the assistant until after they accepted her and that is because you didn't really give enough details you are not in a strong position.
Here are my suggestions. First, I suggest you go to the area supervisor for the school and explain what happened and essentially ask for help but make it clear that if you don't get what your daughter needs you will go higher and will retain a lawyer if necessary.
If that doesn't work you should approach your elected local school trustee.
If that doesn't work you should then retain a lawyer to take this up to the school board and then the ministry and threaten to sue.
And then if necessary you can have the lawyer seek a court order requiring the school to adhere to their offer and enrol your child.
You child has a legal right to an education and a legal right to having her special needs met so long as doing so does not cause the school to suffer undue hardship.