I had a regular work permit in Montreal, and after my spouse arrived later on an open work permit, we went to RAMQ and they enrolled her. We had read on the web that people with open work permits shouldn't be eligible, but they enrolled her anyway. I don't know if there's a difference between the "principal" temporary resident having a work permit vs. study permit, or if we just got lucky and the RAMQ worker didn't know the rules...
Leon's suggestion may be the way to go.
The point here is that they don't mention any case related if the principal applicant is a student. Your case is an "standard" case and the common way to obtain the RAMQ
You could ask the employer to apply for an LMO for you so you can get a regular work permit which would be in your name and the employers and would prove that you are working there.
CIC told me that they can't ensure that, even doing that process, the name of the employer appears because of my previous status. And there's another problem here, because I'm living in Montreal, so I should go abroad to apply, I guess.
It's all so frustrating!
Thank you all for your replies!