Can my employer force me to resign?

The privately owned company I work for in Windsor Ontario Canada, just got a union in. There is no contract at the moment so things have been crazy lately! I am a fulltime employee at the company, but have decided to return to school fulltime. I asked the manager if I could apply for a part time position that had been posted (no one internally applyed therefore they were looking outside) she claimed it shouldn't be a problem. Next thing I know, they started the interviewing process and hired a person with no background! I was told that if I had mentioned it earlier than it would have been fine and they're not going to waste time trainig me when i'm just going to turn around and leave. Then I was told that there are some part time hours in a different department that I currently work saturdays in. ( This deparment is currently short 4 people but the positions have never been posted) Great I thought. Now i'm being told that if I decide to go back to school, I am going to have to resign and I can reapply for any part time position that comes up.(Therefore lose my senority and not to mention they hire from within) The situation is tricky because we don't have a contract yet and the company is making up their owns rules in the meantime!

Gosh, you showed your hand a little too early... I know nothing about Canadian labor laws, but from a strategic point of view I don't immediately see any leverage you have here that can salvage the situation since you've given them the impression that working for them is not your priority.

I doubt they can force you to quit, although they made it clear that they no longer want to invest their resources in you/your employment since they think you've got one foot out the door. Now you have to be super careful not to give them any easy reasons to fire you. You can't be late, look tired, or give any indication that what you do outside of work is in any way having an effect on what you do at work. In fact, you now have to go above and beyond to show your commitment to them to try to help yourself not get forced out.

So as alternatives, I'm wondering... Can you do school part-time without mentioning it to anyone, and continue to work full-time until you have a contract and a firmer understanding of what rights you have under it? Can you postpone school? Can you look for a part-time at another company?

(I'm an owner of a company with just over 100 employees, by the way... so I'm trying to give you a perspective based on how most execs operate.)