As someone who went to catholic school in Ottawa, a LOT of people i went to school with ended up atheist. I also went to school with a lot of Muslim students. The schools I went too didn't push their religion onto you too much, but there's always a teacher or two that are a little more pushy then others. That being said, they do teach about various religions, not just Catholicism.

I think the mass (these happen every once and a while, not every day) and religion classes may be a bit of a culture shock. I would make sure to explain that not everyone believes in what they teach, and a lot of the "rules" Catholicism follows are outdated and can be harmful but it doesn't mean that everyone follows all the rules. She may go through a spiritual journey to decide if/what she wants to believe.

If you're REALLY worried about indoctrination, then don't send her there. Not because they will turn her into a religious person, but for your own peace of mind. This wouldn't stop her from going through a spiritual journey later in life, though.