Take either the PVTA bus (free for UMass students and cheap for everyone else) or the Peter Pan Bus (more expensive) from Amherst to Northhampton, MA. Take the Amtrak from Northampton to Toronto (this will require two or three Amtrak trains; you will have to change trains once or twice).

Option 2:
Take the Peter Pan bus from Amherst to Springfield, MA. Take the Greyhound Lines, Inc., bus from Springfield, MA, to Buffalo, NY. Take the Greyhound Lines of Canada bus from Buffalo, NY, to Toronto.

Option 3:

Take the Peter Pan Bus from Amherst to New York Port Authority Bus Terminal (midtown Manhattan). Take another bus (I think it's either Adirondack Trailways or Greyhound) from New York Port Authority Bus Terminal to Buffalo, NY. Take a third bus (I think it's Greyhound lines of Canada, might be Trailways) from Buffalo, NY, to Toronto.