Buddy its not as bad as it sounds or you are making it sound. I am a frequent flyer...It is what it is....if you are studying in canada one flight wont cause you much problem unless you have a medical condition.
Also, check the fares, baggage requirement, lay over times for your flights...this is more important than the length of the flight. If you have already booked on a good fare, then why do you wanna change it just coz its 15 hour flight.
if its really that big an issue - try india - london - toronto or india - amsterdam - toronto.