If you have the original work and it's more than 50 years old and it is not copyright protected, you can copy it. Almost all publications in circulation are reprints and are copyright protected. No, it's not. In fact, it's illegal to make changes. If it's American, it can be shown in a Canadian classroom.
All foreign work is protected by international agreements.
It may even be more difficult to obtain foreign copyright clearances. I'm a teacher, I can copy anything. You must obey copyright law and you must ensure that your students understand copyright protections. Performances in schools don't require copyright clearances. If the public is invited, if admission is charged, clearance must be obtained.