School Contract issue!!!

I attend school for hair styling, which requires a certain amount of hours according to the Ontario ministry. I had a written contract with my school administrator stating that if i attended a certain amount of hours a week i would be finished school the end of October at the latest, and charged no interest. I attended more hours than were required of me to graduate on time, and now the school has informed me that my contract was made incorrectly and i will have to attend school much longer than i had planned. This will cost me more money in several different ways. I was wondering because i have everything in writing in my contract, does that mean i have to graduate on the date they promised?Are they required to give me those hours? Do i have a legal case here?
I would very much appreciate any advice you could give me and if need be if I have a case and the school still refuses to hold their end of the bargain I would be willing to pay for representation.
Thank you for any response you can give.

You could certainly sue them for any extra time and costs involved. They did give you a contract and you relied on it -- with great time and cost.