Well, unofficially (as in, while I work at Queen's this isn't my thing, and I'm not doing anything more than posting this one comment), I believe that they sometimes do look at the differential for the difference between schools.

You could contact the Commerce Admissions people, explain the situation politely, and ask them how to proceed.

The key is to offer ways for them to verify what you say. Saying 'oh my school is hard' has absolutely no value. Pointing to an official website or putting them in contact with a high level administrator who can provide stats might help (no, not your teacher, that person has a conflict of interest) especially if they can be seen on public sites.

You should explain when more marks will come.

You might also consider an internationally accredited math test, yes, but contact Commerce and ask them.

One thing to remember.... they are getting probably 50 calls a day with some variation on this message, so be polite, prepared, brief, and don't plead or expect to be treated specially. The admissions system is impersonal and so fair, or at least equally unfair to anyone. Making an exception or bending the rules is equivalent to making it unfair. They're not going to do that, and it will probably annoy them if you in any way imply that you want special treatment.